Microbiology Quiz: Specimen Collection
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Specimen CollectionQuestion 1 of 20

A phlebotomist disinfects a patient's arm with chlorhexidine gluconate and allows it to air dry completely before drawing blood for cultures. Just before inserting the needle, the phlebotomist palpates the vein again with a clean, un-gloved index finger. This action most significantly increases the risk of which specific outcome?

Contamination of the blood culture with coagulase-negative staphylococci.
Hemolysis of the sample, leading to interference with automated detection.
Introduction of antiseptic-resistant nosocomial pathogens into the culture.
Inactivation of the culture media broth due to transfer of skin oils.
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Microbiology Quiz

Microbiology Quiz: Specimen Collection

Practice Specimen Collection in Microbiology with focused quiz questions that help you check what you know, review explanations, and build confidence with test-style prompts.

What this quiz covers

This quiz focuses on Specimen Collection, giving you a quick way to practice the rules, question types, and explanations that matter most for Microbiology.

How to use this quiz

Try each quiz question before looking at the correct answer. Use the explanations to review missed ideas, then come back to similar questions until the pattern feels familiar.

All questions

Question 1

A phlebotomist disinfects a patient's arm with chlorhexidine gluconate and allows it to air dry completely before drawing blood for cultures. Just before inserting the needle, the phlebotomist palpates the vein again with a clean, un-gloved index finger. This action most significantly increases the risk of which specific outcome?

  1. Contamination of the blood culture with coagulase-negative staphylococci. (correct answer)
  2. Hemolysis of the sample, leading to interference with automated detection.
  3. Introduction of antiseptic-resistant nosocomial pathogens into the culture.
  4. Inactivation of the culture media broth due to transfer of skin oils.
Explanation: The correct answer is A. Re-palpating a disinfected venipuncture site with a non-sterile finger is a major breach of aseptic technique. This action transfers skin flora from the phlebotomist's finger onto the prepared site. Coagulase-negative staphylococci (e.g., Staphylococcus epidermidis) are the most common microorganisms found on the skin and are, therefore, the most likely contaminants to be introduced into the blood culture bottle via this error. Hemolysis (B) is caused by mechanical trauma during the draw, not palpation. While nosocomial pathogens (C) could be present, the most probable contaminants are the person's own normal flora. Skin oils (D) do not inactivate culture media.

Question 2

A surgeon collects a sample from a deep pelvic abscess for anaerobic culture.

Which of the following collection and transport methods is optimal for ensuring the viability of obligate anaerobic organisms?

  1. Aspirating purulent material into a syringe, carefully expelling any trapped air, and injecting it into an anaerobic transport vial. (correct answer)
  2. Collecting a piece of the abscess wall (tissue biopsy) and placing it in a sterile container with a few drops of sterile saline.
  3. Using a Dacron swab to absorb as much purulent material as possible and placing it into an anaerobic transport pouch.
  4. Swabbing the abscess generously and placing the swab into a tube containing Stuart's transport medium.
Explanation: The correct answer is A. The most critical factor for recovering obligate anaerobes is minimizing exposure to oxygen. Aspirating fluid or pus directly into a syringe allows for the collection of a large volume with minimal air contact. Expelling any residual air and then injecting the sample into a specialized anaerobic transport medium (which is oxygen-free and may have an indicator) is the best practice. Distractor B is a good method, but less ideal than an aspirate in an anaerobic vial, as the container still contains air. Distractor C is suboptimal because swabs collect less volume and have a high surface-area-to-volume ratio, increasing oxygen exposure. Distractor D is incorrect because Stuart's medium is a general bacterial transport medium and is not suitable for preserving strict anaerobes.

Question 3

A laboratory receives several specimens accompanied by requisitions for anaerobic culture. Based on established specimen rejection criteria, which of the following specimens is inherently unsuitable for anaerobic culture and should be rejected?

  1. Aspirated fluid from a lung abscess collected via transtracheal aspiration.
  2. A tissue biopsy from a non-healing diabetic foot ulcer obtained after debridement.
  3. A midstream clean-catch urine sample from a patient with a suspected complicated UTI. (correct answer)
  4. Peritoneal fluid collected during surgery for a ruptured appendix.
Explanation: The correct answer is C. Anaerobic culture should only be performed on specimens from normally sterile sites. A voided urine sample, even if collected 'clean-catch', is always contaminated with the normal anaerobic flora of the urethra. Therefore, culturing it for anaerobes is clinically meaningless and will always yield growth that cannot be distinguished from infection. The other specimens (A, B, D) are all collected from sites that are either sterile or, in the case of the debrided ulcer, represent a deep tissue sample where anaerobes are clinically significant pathogens.

Question 4

A nurse is preparing to collect a urine specimen for culture from a patient with a long-term indwelling Foley catheter and a closed drainage system. Which of the following actions represents the correct procedure that minimizes the risk of both specimen contamination and patient infection?

  1. Disconnecting the catheter from the drainage tube and allowing fresh urine to flow into a sterile cup.
  2. Withdrawing urine from the collection bag using a sterile syringe, as this provides an averaged sample.
  3. Removing the old catheter, having the patient provide a midstream specimen, and then inserting a new catheter.
  4. Clamping the tubing distal to the collection port, waiting 15-20 minutes, disinfecting the port, and aspirating urine with a sterile syringe. (correct answer)
Explanation: The correct answer is D. This describes the complete, correct, and aseptic procedure. Clamping the tube allows fresh urine from the bladder to collect in the tubing. The collection port is designed for this purpose and must be disinfected before aspiration to prevent contamination of the specimen. This method maintains the closed system, protecting the patient from infection, while providing a specimen that accurately reflects the urine in the bladder. Distractor A is incorrect because it breaks the closed system, creating a major risk of introducing bacteria into the bladder. Distractor B is incorrect because urine in the collection bag is stagnant and does not represent an active infection; it is considered a contaminated source. Distractor C is unnecessarily invasive and impractical as a routine collection method.

Question 5

A surgeon collects a single, small tissue biopsy from a suspected deep wound infection. The sample must be analyzed by both the microbiology lab for culture and the pathology lab for histology. Which of the following actions would irreversibly compromise the viability of microorganisms for culture?

  1. Placing the entire specimen into a container filled with 10% formalin. (correct answer)
  2. Dividing the specimen on a sterile field and sending one half in saline and the other half in formalin.
  3. Wrapping the specimen in sterile, saline-moistened gauze before placing it in a dry sterile container.
  4. Delaying transport to the laboratory for up to 2 hours at room temperature.
Explanation: The correct answer is A. Formalin is a potent fixative used for preserving tissue structure for histopathological examination. It is bactericidal and will kill all microorganisms in the specimen, making it completely unsuitable for culture. If the entire single specimen is placed in formalin, no microbiological analysis is possible. B describes the correct procedure when a single specimen is collected for both purposes. C is a correct way to prepare a specimen for culture to prevent it from drying out. D is a suboptimal delay but would not irreversibly destroy all microorganisms like formalin does.

Question 6

For the diagnosis of many viral respiratory infections, such as influenza and RSV, nasopharyngeal (NP) swabs are often preferred over oropharyngeal (OP) swabs. What is the primary scientific principle that justifies this preference?

  1. NP swabs are less likely to be contaminated with oral bacteria, which can inhibit the polymerase chain reaction (PCR) assay.
  2. The procedure for collecting an NP swab is less technically challenging, leading to more consistent sample quality.
  3. The ciliated columnar epithelial cells of the nasopharynx are the primary site of replication for many respiratory viruses, yielding a higher viral load. (correct answer)
  4. NP swabs are more comfortable for the patient, which encourages better compliance and a more thorough collection.
Explanation: The correct answer is C. The anatomical and physiological reason for the superior sensitivity of NP swabs for many viruses is that they sample the nasopharynx, an area rich in ciliated columnar epithelial cells. These cells are the preferred target for replication by viruses like influenza, RSV, and coronaviruses. A properly collected NP swab harvests these infected cells, resulting in a higher concentration of viral particles and nucleic acid, which leads to greater diagnostic sensitivity. While PCR inhibition (A) is a real concern, the primary advantage is the higher viral yield. NP swabs are generally considered more technically challenging (B) and less comfortable (D) than OP swabs.

Question 7

A patient with persistent diarrhea is instructed to provide three stool specimens for ova and parasite (O&P) examination. The patient collects three separate aliquots from a single large bowel movement and submits them on the same day.

What is the primary reason this collection method is considered inadequate for a comprehensive O&P examination?

  1. The shedding of many protozoan cysts and helminth eggs is intermittent and may not be detected in a single fecal event. (correct answer)
  2. Trophozoites are fragile and will lyse in the later samples due to the time delay from a single large collection.
  3. The concentration of parasites is highest in the first part of the stool, making subsequent aliquots from the same movement redundant.
  4. The required preservative-to-stool ratio cannot be accurately achieved when preparing three vials from one sample.
Explanation: The correct answer is A. The primary rationale for collecting three separate stool samples over several days (e.g., every other day) is that many intestinal parasites, particularly protozoa like Giardia lamblia and Entamoeba histolytica, are not shed continuously. Their numbers in stool can fluctuate daily. Collecting samples from different days significantly increases the probability of detecting the organism if an infection is present. Collecting three samples from one bowel movement does not account for this intermittent shedding and offers little diagnostic advantage over collecting a single sample. While trophozoite viability (B) is a concern related to transport time, it's not the reason for collecting on different days. C is incorrect; parasite distribution is not necessarily concentrated in one part of the stool. D is a logistical issue but not the core parasitological principle.

Question 8

A quantitative culture of a bronchoalveolar lavage (BAL) fluid specimen is performed. The laboratory's criterion for a significant result indicating infection is ≥104≥10^4 CFU/mL. The culture yields 6×1036 \times 10^3 CFU/mL of Pseudomonas aeruginosa. The cytospin analysis of the fluid shows numerous neutrophils and very few (<1%) squamous epithelial cells.

What is the most accurate interpretation of these combined findings?

  1. The patient has a definite P. aeruginosa pneumonia, as indicated by the presence of neutrophils.
  2. The specimen is of good quality, but the bacterial count is below the diagnostic threshold, likely representing colonization or contamination. (correct answer)
  3. The specimen is contaminated with upper airway flora, and the results are therefore uninterpretable.
  4. The result is borderline, so the P. aeruginosa should be reported as a potential pathogen requiring clinical correlation.
Explanation: The correct answer is B. This question requires integrating two pieces of information. First, the cellular analysis shows very few squamous epithelial cells, which indicates that the BAL specimen is of high quality and not significantly contaminated by upper airway secretions. Second, the quantitative culture result (6×1036 \times 10^3 CFU/mL) is below the established diagnostic cutoff of 10410^4 CFU/mL. Therefore, despite the good quality of the specimen, the bacterial growth does not meet the criterion for infection and most likely represents colonization of the lower airways or minor contamination during the bronchoscopy procedure. Distractor A is incorrect because the colony count is too low. Distractor C is incorrect because the cellular findings indicate a good quality, not contaminated, specimen. Distractor D is a possible report, but B is a more precise interpretation of the data against the established threshold.

Question 9

A hospital's infection control team notes a sudden increase in the number of blood cultures positive for Staphylococcus epidermidis originating exclusively from the emergency department (ED). All isolates exhibit identical antibiotic susceptibility patterns. An audit of ED procedures is initiated.

Given this scenario, which of the following represents the most probable single point of failure in the specimen collection process leading to this cluster of pseudobacteremia?

  1. Systemic contamination of the laboratory's automated blood culture incubation system.
  2. Consistent failure to disinfect the rubber septa of the culture bottles prior to inoculation.
  3. Inadequate skin antisepsis technique, such as insufficient drying time or re-palpation of the site after disinfection. (correct answer)
  4. Routinely drawing blood for cultures through pre-existing peripheral intravenous catheters.
Explanation: The correct answer is C. Staphylococcus epidermidis is the most common component of normal skin flora and the most frequent contaminant in blood cultures. A cluster of identical isolates localized to one department strongly suggests a procedural error in collection. Inadequate skin antisepsis, such as not allowing the antiseptic to dry or re-palpating the disinfected site, is a common error that directly reintroduces skin flora onto the puncture site, leading to contamination of the sample. Distractor A is less likely because contamination in the lab would affect samples from all hospital departments, not just the ED. Distractor B is a possible cause of contamination, but improper skin preparation (C) is a more frequent and widespread error that can explain a larger cluster. Distractor D is a known risk factor for contamination, but it is a less specific error than inadequate skin antisepsis, which is a fundamental and critical step for all venipunctures for culture.

Question 10

A courier transports several specimens from an outpatient clinic to the hospital laboratory. The transport box is maintained with cold packs at approximately 4°C. The batch includes a urine sample for culture, a CSF sample for bacterial meningitis workup, a throat swab in transport media, and a stool sample in a preservative vial.

The transport temperature of 4°C is most likely to cause a false-negative result for which of these specimens?

  1. Urine sample for culture.
  2. Stool sample in preservative.
  3. Throat swab for S. pyogenes.
  4. CSF for bacterial culture. (correct answer)
Explanation: The correct answer is D. CSF specimens for bacterial culture should be kept at room temperature and transported to the lab immediately. Refrigeration can kill or inhibit the growth of fastidious pathogens commonly associated with meningitis, such as Neisseria meningitidis and Haemophilus influenzae, leading to a false-negative culture. In contrast, urine (A) should be refrigerated to prevent the overgrowth of contaminants. Stool in preservative (B) is stable at various temperatures, including refrigeration. Throat swabs for Group A Strep (C) are also stable when refrigerated.

Question 11

A lumbar puncture is performed on a patient with suspected bacterial meningitis. Four tubes of cerebrospinal fluid (CSF) are collected sequentially. The tubes are numbered 1, 2, 3, and 4 in order of collection.

To minimize the impact of potential contaminants on diagnostic accuracy, which tube should be prioritized for microbiology culture and Gram stain, and for what reason?

  1. Tube 1, as it contains the highest concentration of pathogens before the CSF pressure equalizes.
  2. Tube 2 or 3, because the initial flow of CSF in Tube 1 may be contaminated with skin flora from the needle insertion. (correct answer)
  3. Tube 4, as it is the least likely to contain red blood cells from a traumatic tap, which can interfere with testing.
  4. Any tube is equally acceptable, provided they were all collected aseptically into sterile tubes.
Explanation: The correct answer is B. Standard procedure dictates using the second or third tube of CSF for microbiology. The rationale is that the spinal needle can carry a small plug of skin and superficial tissue into the spinal canal upon insertion. The initial flow of CSF (collected in Tube 1) flushes out these potential contaminants. By using a subsequent tube, the risk of misinterpreting skin flora as the cause of meningitis is significantly reduced. Distractor A is based on a false premise; pathogen concentration is generally homogenous. Distractor C describes the rationale for using Tube 4 for cell counts (to differentiate a traumatic tap from subarachnoid hemorrhage), but for microbiology, avoiding skin contamination is the primary concern. Distractor D ignores the well-established procedural nuance designed to mitigate collection-related contamination.

Question 12

A hospital's infection control team notes a sudden increase in the number of blood cultures positive for Staphylococcus epidermidis originating exclusively from the emergency department (ED). All isolates exhibit identical antibiotic susceptibility patterns. An audit of ED procedures is initiated.

Given this scenario, which of the following represents the most probable single point of failure in the specimen collection process leading to this cluster of pseudobacteremia?

  1. Systemic contamination of the laboratory's automated blood culture incubation system.
  2. Consistent failure to disinfect the rubber septa of the culture bottles prior to inoculation.
  3. Inadequate skin antisepsis technique, such as insufficient drying time or re-palpation of the site after disinfection. (correct answer)
  4. Routinely drawing blood for cultures through pre-existing peripheral intravenous catheters.
Explanation: The correct answer is C. Staphylococcus epidermidis is the most common component of normal skin flora and the most frequent contaminant in blood cultures. A cluster of identical isolates localized to one department strongly suggests a procedural error in collection. Inadequate skin antisepsis, such as not allowing the antiseptic to dry or re-palpating the disinfected site, is a common error that directly reintroduces skin flora onto the puncture site, leading to contamination of the sample. Distractor A is less likely because contamination in the lab would affect samples from all hospital departments, not just the ED. Distractor B is a possible cause of contamination, but improper skin preparation (C) is a more frequent and widespread error that can explain a larger cluster. Distractor D is a known risk factor for contamination, but it is a less specific error than inadequate skin antisepsis, which is a fundamental and critical step for all venipunctures for culture.

Question 13

A clean-catch midstream urine sample from an asymptomatic 78-year-old female resident of a nursing home is submitted for culture. The laboratory report shows the following: >100,000 CFU/mL Escherichia coli, >100,000 CFU/mL Enterococcus faecalis, and 20,000 CFU/mL Candida albicans. Microscopic examination reveals >25 squamous epithelial cells per low-power field (LPF).

What is the most appropriate interpretation of these results and the recommended course of action?

  1. The patient has a significant polymicrobial urinary tract infection and should be treated with broad-spectrum antibiotics.
  2. The specimen is grossly contaminated with perineal flora; a new specimen should be collected after providing proper collection instructions. (correct answer)
  3. The E. coli and Enterococcus are significant, but the Candida represents colonization and does not require treatment.
  4. Request an indwelling catheter specimen to bypass contamination, as the high colony counts are clinically concerning.
Explanation: The correct answer is B. The presence of >25 squamous epithelial cells/LPF is a key indicator that the specimen is not a true 'clean-catch' and is contaminated with flora from the external genitalia. The growth of three or more different organisms (polymicrobial growth), especially in an asymptomatic patient, further supports the conclusion of contamination rather than a true infection. Therefore, the results are uninterpretable, and the most appropriate action is to request a new, properly collected specimen. Distractor A is incorrect because treating based on a contaminated sample can lead to unnecessary antibiotic use. Distractor C makes an arbitrary distinction between the organisms without addressing the primary evidence of contamination. Distractor D suggests an invasive procedure, which is not the appropriate first step; re-education and attempting another clean-catch collection should precede catheterization.

Question 14

A Gram stain is performed on an expectorated sputum sample from a patient with pneumonia. Microscopic examination reveals >25 polymorphonuclear leukocytes (PMNs) per low-power field (LPF) and <10 squamous epithelial cells (SECs) per LPF. Numerous Gram-positive cocci in clusters and Gram-negative rods are observed.

Based on these microscopic findings, what is the most appropriate action for the laboratory technologist to take?

  1. Reject the specimen due to contamination with oropharyngeal flora and request a new sample.
  2. Proceed with a full culture workup as the specimen is of high quality and represents lower respiratory secretions. (correct answer)
  3. Report 'mixed oral flora present' and recommend clinical correlation for treatment.
  4. Perform a quantitative culture to determine the predominant organism, as multiple morphologies are present.
Explanation: The correct answer is B. The microscopic findings meet the criteria for a high-quality sputum specimen (specifically, Bartlett's or similar criteria), indicating it originates from the lower respiratory tract rather than being saliva. The presence of many PMNs (an indicator of inflammation) and few SECs (an indicator of minimal oral contamination) confirms its acceptability. Despite the presence of multiple bacterial morphologies, the quality of the specimen warrants a full culture workup to identify the potential pathogen(s). Distractor A is incorrect because the cellular makeup explicitly indicates low contamination. Distractor C would be the correct action for a poor-quality specimen (many SECs, few PMNs). Distractor D is incorrect as quantitative cultures are not standard for routine sputum and are typically reserved for specimens like bronchoalveolar lavage (BAL) fluid.

Question 15

A surgeon collects a sample from a deep pelvic abscess for anaerobic culture.

Which of the following collection and transport methods is optimal for ensuring the viability of obligate anaerobic organisms?

  1. Aspirating purulent material into a syringe, carefully expelling any trapped air, and injecting it into an anaerobic transport vial. (correct answer)
  2. Collecting a piece of the abscess wall (tissue biopsy) and placing it in a sterile container with a few drops of sterile saline.
  3. Using a Dacron swab to absorb as much purulent material as possible and placing it into an anaerobic transport pouch.
  4. Swabbing the abscess generously and placing the swab into a tube containing Stuart's transport medium.
Explanation: The correct answer is A. The most critical factor for recovering obligate anaerobes is minimizing exposure to oxygen. Aspirating fluid or pus directly into a syringe allows for the collection of a large volume with minimal air contact. Expelling any residual air and then injecting the sample into a specialized anaerobic transport medium (which is oxygen-free and may have an indicator) is the best practice. Distractor B is a good method, but less ideal than an aspirate in an anaerobic vial, as the container still contains air. Distractor C is suboptimal because swabs collect less volume and have a high surface-area-to-volume ratio, increasing oxygen exposure. Distractor D is incorrect because Stuart's medium is a general bacterial transport medium and is not suitable for preserving strict anaerobes.

Question 16

A patient with persistent diarrhea is instructed to provide three stool specimens for ova and parasite (O&P) examination. The patient collects three separate aliquots from a single large bowel movement and submits them on the same day.

What is the primary reason this collection method is considered inadequate for a comprehensive O&P examination?

  1. The shedding of many protozoan cysts and helminth eggs is intermittent and may not be detected in a single fecal event. (correct answer)
  2. Trophozoites are fragile and will lyse in the later samples due to the time delay from a single large collection.
  3. The concentration of parasites is highest in the first part of the stool, making subsequent aliquots from the same movement redundant.
  4. The required preservative-to-stool ratio cannot be accurately achieved when preparing three vials from one sample.
Explanation: The correct answer is A. The primary rationale for collecting three separate stool samples over several days (e.g., every other day) is that many intestinal parasites, particularly protozoa like Giardia lamblia and Entamoeba histolytica, are not shed continuously. Their numbers in stool can fluctuate daily. Collecting samples from different days significantly increases the probability of detecting the organism if an infection is present. Collecting three samples from one bowel movement does not account for this intermittent shedding and offers little diagnostic advantage over collecting a single sample. While trophozoite viability (B) is a concern related to transport time, it's not the reason for collecting on different days. C is incorrect; parasite distribution is not necessarily concentrated in one part of the stool. D is a logistical issue but not the core parasitological principle.

Question 17

A nurse is preparing to collect a urine specimen for culture from a patient with a long-term indwelling Foley catheter and a closed drainage system. Which of the following actions represents the correct procedure that minimizes the risk of both specimen contamination and patient infection?

  1. Disconnecting the catheter from the drainage tube and allowing fresh urine to flow into a sterile cup.
  2. Withdrawing urine from the collection bag using a sterile syringe, as this provides an averaged sample.
  3. Removing the old catheter, having the patient provide a midstream specimen, and then inserting a new catheter.
  4. Clamping the tubing distal to the collection port, waiting 15-20 minutes, disinfecting the port, and aspirating urine with a sterile syringe. (correct answer)
Explanation: The correct answer is D. This describes the complete, correct, and aseptic procedure. Clamping the tube allows fresh urine from the bladder to collect in the tubing. The collection port is designed for this purpose and must be disinfected before aspiration to prevent contamination of the specimen. This method maintains the closed system, protecting the patient from infection, while providing a specimen that accurately reflects the urine in the bladder. Distractor A is incorrect because it breaks the closed system, creating a major risk of introducing bacteria into the bladder. Distractor B is incorrect because urine in the collection bag is stagnant and does not represent an active infection; it is considered a contaminated source. Distractor C is unnecessarily invasive and impractical as a routine collection method.

Question 18

A throat swab is collected from a child with acute pharyngitis for a Group A Streptococcus (GAS) rapid antigen detection test (RADT), which is negative. A backup culture is also negative. Two days later, the child's condition worsens, and a second swab collected by a different clinician is positive on both RADT and culture.

Which specimen collection error is the most frequent cause of initial false-negative results in this scenario?

  1. Failure to place the swab in transport medium, allowing the specimen to dry out before testing.
  2. Using a calcium alginate swab, which can be inhibitory to some respiratory pathogens.
  3. Swabbing the tongue, uvula, or buccal mucosa instead of vigorously swabbing both tonsillar pillars and the posterior pharynx. (correct answer)
  4. Collecting the sample after the patient has recently used an antiseptic mouthwash.
Explanation: The correct answer is C. The highest concentration of Group A Streptococcus is found on the surface of the tonsils and the posterior pharyngeal wall. The most common collection error leading to false negatives is inadequate technique, where the swab is contaminated with saliva and oral flora from other parts of the mouth without making good contact with the infected areas. This results in an insufficient bacterial load (antigen) for detection. While D is a possible cause, it's a less frequent error than poor swabbing technique. A and B are also potential issues but are less common sources of error in modern practice than simply failing to swab the correct location.

Question 19

A urine specimen collected from an 11-month-old infant via an adhesive collection bag grows 60,000 CFU/mL of Klebsiella pneumoniae. The urinalysis shows no white blood cells or nitrites. What is the primary challenge in the clinical interpretation of this finding?

  1. The colony count is below the traditional 100,000 CFU/mL threshold, definitively ruling out a urinary tract infection (UTI).
  2. The adhesive bag collection method has a very high rate of contamination from perineal flora, making the result difficult to interpret. (correct answer)
  3. Klebsiella pneumoniae is an environmental organism and is not a recognized urinary pathogen in this age group.
  4. The absence of pyuria and nitrites in the urinalysis reliably excludes a UTI, regardless of the culture result.
Explanation: The correct answer is B. Urine collection bags are notoriously prone to contamination from skin, perineal, and fecal bacteria. It is very difficult to distinguish a true, low-count bacteriuria from contamination when this method is used. Any growth from a bagged specimen must be interpreted with extreme caution and in the context of strong clinical symptoms. Distractor A is incorrect because the 100,000 CFU/mL threshold is not absolute, especially in children or symptomatic patients; lower counts can be significant. Distractor C is false; Klebsiella is a common uropathogen. Distractor D is incorrect because infants may not always exhibit pyuria or have nitrite-positive urine during a UTI, so a negative urinalysis does not definitively rule it out.

Question 20

A phlebotomist disinfects a patient's arm with chlorhexidine gluconate and allows it to air dry completely before drawing blood for cultures. Just before inserting the needle, the phlebotomist palpates the vein again with a clean, un-gloved index finger. This action most significantly increases the risk of which specific outcome?

  1. Contamination of the blood culture with coagulase-negative staphylococci. (correct answer)
  2. Hemolysis of the sample, leading to interference with automated detection.
  3. Introduction of antiseptic-resistant nosocomial pathogens into the culture.
  4. Inactivation of the culture media broth due to transfer of skin oils.
Explanation: The correct answer is A. Re-palpating a disinfected venipuncture site with a non-sterile finger is a major breach of aseptic technique. This action transfers skin flora from the phlebotomist's finger onto the prepared site. Coagulase-negative staphylococci (e.g., Staphylococcus epidermidis) are the most common microorganisms found on the skin and are, therefore, the most likely contaminants to be introduced into the blood culture bottle via this error. Hemolysis (B) is caused by mechanical trauma during the draw, not palpation. While nosocomial pathogens (C) could be present, the most probable contaminants are the person's own normal flora. Skin oils (D) do not inactivate culture media.