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Interpreting Lab Values
And Diagnostics

Learn to read, compare, and act on common lab results so you can spot what is normal, what is critical, and what to do next.

NCLEX-RN

What Is a Lab Value?

A lab value is a measured amount of a substance in the body, such as in blood or urine. Each value is compared to a reference range — the band of results considered normal for healthy people.

Reading a Reference Range

Normal range Low limit High limit Below = Low Above = High Patient result inside range = normal

Why It Matters

Detect Problems
Labs reveal organ damage or imbalance before symptoms appear.
Guide Treatment
Drug doses (like insulin or warfarin) are adjusted from lab results.
Ensure Safety
Critical values demand quick action and prescriber notification.

The Core Rule

Key Normal Ranges

LabNormal RangeWatch For
Potassium (K⁺)\( 3.5 - 5.0 \) mEq/LCardiac arrhythmias
Sodium (Na⁺)\( 135 - 145 \) mEq/LNeuro changes
Glucose (fasting)\( 70 - 110 \) mg/dLHypo/hyperglycemia
Hemoglobin (M)\( 14 - 18 \) g/dLAnemia / bleeding
WBC\( 5{,}000 - 10{,}000 \)/µLInfection

Abnormal vs. Critical

AbnormalCritical
SeverityOutside rangeLife-threatening
UrgencyMonitor / trendAct now
ActionReassess, documentNotify prescriber STAT
Example K⁺\( 5.4 \) mEq/L\( 6.8 \) mEq/L

How to Interpret a Result

1
Identify the test and recall its reference range.
2
Compare the patient value to the range — above, below, or within?
3
Judge severity — is this mildly abnormal or critical?
4
Correlate with the patient's symptoms and medications.
5
Act and document — notify, intervene, and record the report.

Worked Example: Potassium

\( 3.5 - 5.0 \)
\( 5.9 \) mEq/L
Above range = high Patient K⁺ result plotted against the normal band

Worked Example: Glucose

\( 70 - 110 \) mg/dL
\( 52 \) mg/dL
Below range = low Fasting glucose below the normal band

Worked Example: Trending Values

18 8 Time (days) Hgb g/dL normal band

Diagnostics Beyond Labs

ECG
Reads heart rhythm — vital with potassium or calcium imbalance.
ABG
Shows pH and gases to judge acid-base and oxygenation status.
Imaging
X-ray, CT, MRI confirm structural findings labs cannot show.

Common Mistakes

✕ Wrong Direction
Confusing "high" with "low" — leads to the wrong intervention.
✕ Ignoring Trends
Treating one in-range value as safe while it steadily worsens.
✕ Missing Critical
Failing to notify the prescriber for a critical value.
✕ Skipping Context
Reading numbers without the patient's symptoms or meds.

Quick Summary

StepAsk Yourself
IdentifyWhich test is this and what is normal?
CompareIs the value high, low, or within range?
SeverityMildly abnormal or critical?
CorrelateDo symptoms and meds match the result?
ActIntervene, notify, and document.

Question 1 of 5

A client's potassium is \( 6.5 \) mEq/L (normal \( 3.5 - 5.0 \)). What does this represent?

Question 2 of 5

A fasting glucose is \( 58 \) mg/dL (normal \( 70 - 110 \)) in an alert client. What is the priority?

Question 3 of 5

Which best defines a critical value?

Question 4 of 5

A client's hemoglobin trends 16 → 13 → 10 g/dL over three days. What should the nurse suspect?

Question 5 of 5

An elevated WBC count (above \( 10{,}000 \)/µL) most commonly indicates what?

You've Got the Framework

You can now identify a test, compare it to its range, judge severity, correlate with the patient, and act. That five-step path turns any lab value into a clear nursing decision.