Lab Values to Know Cold for the NCLEX

Fundamentals · Vital Signs & Assessment·Updated 2026-10-28

What it is

NCLEX lab questions rarely ask for a number alone; they ask which result is dangerous for this client right now and what the nurse does first. A value is interpreted against the client's baseline, trend, diagnosis and medications. The results that kill fastest are the ones that disturb cardiac conduction, oxygenation, brain glucose or clotting, so potassium, glucose, arterial blood gases, platelets and the INR come first.

What the NCLEX focuses on

Hyperkalemia shows peaked T waves and a widening QRS; hypokalemia shows flat T waves and U waves, and it increases digoxin toxicity.
ACE inhibitors, ARBs, spironolactone and kidney failure raise potassium; loop and thiazide diuretics lower potassium and magnesium.
IV potassium is never given by IV push; it is diluted and infused by pump, no faster than 10 mEq/hr on a general unit.
Serum creatinine is the most specific indicator of kidney function; BUN also rises with dehydration, GI bleeding and high protein intake.
A sudden fall in hemoglobin after surgery signals bleeding; troponin is the preferred marker of myocardial injury.
Therapeutic drug levels: digoxin 0.5 to 2.0 ng/mL, lithium 0.6 to 1.2 mEq/L (maintenance), phenytoin 10 to 20 mcg/mL.
A potassium result from a hemolyzed or difficult draw is not ignored: obtain an ECG and redraw promptly.

Priority nursing actions

Assess the client: symptoms, vital signs, and cardiac rhythm for potassium or calcium changes.
Treat a life-threatening value per protocol at once (glucose below 70 mg/dL first, before calling).
Hold the drug that worsens the value (warfarin with a high INR, potassium supplement with hyperkalemia).
Notify the provider using SBAR; read back any critical value received by telephone.
Recheck the value and the client after treatment.

Numbers to know

Lab	Normal or target	Why it matters
Potassium	3.5 to 5.0 mEq/L (3.5 to 5.0 mmol/L)	Both extremes cause dysrhythmias
Sodium	136 to 145 mEq/L (136 to 145 mmol/L)	Abnormal values cause confusion and seizures
Fasting glucose	70 to 99 mg/dL (3.9 to 5.5 mmol/L)	Below 70 mg/dL is hypoglycemia: treat first
Creatinine	0.6 to 1.2 mg/dL (53 to 106 micromol/L)	Rising value means falling kidney function
Platelets	150,000 to 400,000/mm3 (150 to 400 x 10^9/L)	Below 50,000/mm3 raises bleeding risk; bleeding precautions
WBC	4,000 to 11,000/mm3 (4.0 to 11.0 x 10^9/L)	ANC below 500/mm3 is severe neutropenia
INR on warfarin	2.0 to 3.0 for most indications	Above range: hold warfarin, assess for bleeding, report
Arterial blood gas	pH 7.35 to 7.45; PaCO2 35 to 45 mm Hg; HCO3 22 to 26 mEq/L (22 to 26 mmol/L)	Identifies acid-base imbalance

Common exam traps

Choosing to recheck an INR of 6 at the next routine draw: wrong because bleeding risk is immediate; hold warfarin and report now.
Choosing BUN as the best indicator of kidney function: wrong because creatinine is more specific.
Choosing a chronic, stable abnormal result over a new life-threatening one: wrong because potassium 6.8 mEq/L with peaked T waves comes first.
Choosing to give oral potassium "to be safe" with a potassium of 5.4 mEq/L on spironolactone: wrong because it worsens hyperkalemia.
Choosing to call the provider before treating a glucose of 48 mg/dL: wrong because hypoglycemia is treated first per protocol.

What to teach

Keep every scheduled INR or drug-level appointment and report bleeding, black stools or dark urine.
With a potassium-wasting diuretic, report muscle weakness, cramps or palpitations.
Avoid salt substitutes when taking an ACE inhibitor, ARB or spironolactone.

Try 2 NCLEX-style questions on this topic

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Question 1 · QID 35052629

The nurse receives morning laboratory results for four clients. Which client should the nurse assess first?

A. Client with eGFR 54 scheduled for outpatient follow-up
B. Client with creatinine 1.3 mg/dL after starting lisinopril
C. Client with potassium 6.8 mEq/L and an ECG showing peaked T waves
D. Client with BUN 32 mg/dL receiving IV fluids for dehydration

Question 2 · QID 93557A53

A nurse is reviewing results for several clients. For each result, indicate whether it requires immediate follow-up or can be addressed at routine timing.

For each laboratory result, indicate whether it requires immediate follow-up or can be addressed at routine timing.

	Requires immediate follow-up	Can be addressed at routine timing
INR of 6.2 in a client on warfarin for atrial fibrillation
Fasting blood glucose of 98 mg/dL in a client not on TPN
INR of 2.4 in a client on warfarin for atrial fibrillation
Blood glucose of 48 mg/dL in a client whose TPN was just interrupted

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More tips on Fundamentals

Isolation Precautions and PPE: What the NCLEX TestsMatch each infection to airborne, droplet or contact precautions, put PPE on and take it off in the right order, and avoid the classic transport traps.Fall Prevention and Restraint Safety: What the NCLEX TestsSpot fall risk, apply universal fall precautions, try alternatives before restraints, and apply, monitor and release restraints safely.Enteral Feeding and Aspiration Prevention: What the NCLEX TestsVerify tube placement, keep the head of the bed up, recognize aspiration and intolerance, and give medications through a feeding tube safely.

Sources

Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Fluid, Electrolyte, and Acid-Base Imbalances.
Harding MM, Kwong J, Hagler D, Reinisch C. Lewis's Medical-Surgical Nursing: Assessment and Management of Clinical Problems. 12th ed. Elsevier; 2023; Chapter: Acute Kidney Injury and Chronic Kidney Disease.
Pagana KD, Pagana TJ, Pagana TN. Mosby's Manual of Diagnostic and Laboratory Tests. 7th ed. Elsevier; 2022.

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