NCLEX Lab Values Cheat Sheet: Every Critical Value You Must Know

Lab values are a fundamental part of nursing practice, and they're a favorite topic on the NCLEX. You need to know not just the normal ranges, but also what to do when a value is abnormal. This cheat sheet covers every critical lab value you need to know for the NCLEX, organized by category for easy reference.

Electrolytes

Lab	Normal Range	Critical Value	Nursing Action
Sodium (Na+)	135-145 mEq/L	<120 or >155 mEq/L	Low (Hyponatremia): Neurological changes (confusion, seizures). Monitor neuro status, restrict fluids if indicated. High (Hypernatremia): Thirst, agitation. Administer hypotonic fluids slowly.
Potassium (K+)	3.5-5.0 mEq/L	<3.0 or >6.0 mEq/L	Low (Hypokalemia): Muscle weakness, cardiac arrhythmias. Administer potassium supplements, monitor ECG. High (Hyperkalemia): Cardiac arrest risk! Hold potassium, administer calcium gluconate, insulin+glucose, kayexalate. Monitor ECG.
Calcium (Ca2+)	8.5-10.5 mg/dL	<6.5 or >13 mg/dL	Low (Hypocalcemia): Chvostek's/Trousseau's signs, tetany. Administer calcium supplements, seizure precautions. High (Hypercalcemia): "Stones, bones, groans, psychiatric overtones." Hydrate, administer loop diuretics.
Magnesium (Mg2+)	1.5-2.5 mEq/L	<1.0 or >4.0 mEq/L	Low (Hypomagnesemia): Similar to hypocalcemia. Administer magnesium supplements. High (Hypermagnesemia): Loss of deep tendon reflexes, respiratory depression. Hold magnesium, administer calcium gluconate.

Complete Blood Count (CBC)

Lab	Normal Range	Critical Value	Nursing Action
Hemoglobin (Hgb)	M: 13.5-17.5 g/dL F: 12.0-15.5 g/dL	<7.0 g/dL	Assess for anemia symptoms (fatigue, pallor). Prepare for possible blood transfusion.
Hematocrit (Hct)	M: 41-53% F: 36-46%	<21% or >60%	Low: Anemia. High: Polycythemia, dehydration.
White Blood Cells (WBC)	5,000-10,000/mm³	<2,000 or >30,000/mm³	Low: Neutropenia (infection risk). Implement neutropenic precautions. High: Infection, inflammation, leukemia.
Platelets	150,000-400,000/mm³	<20,000/mm³	High risk of bleeding. Implement bleeding precautions (soft toothbrush, electric razor, avoid IM injections).

Coagulation Studies

Lab	Normal Range	Critical Value	Nursing Action
Prothrombin Time (PT)	11-13.5 seconds	>20 seconds	Monitor for bleeding. Often used to monitor warfarin therapy.
International Normalized Ratio (INR)	0.8-1.2	>3.0-4.5 (therapeutic for warfarin)	Therapeutic range for warfarin is usually 2.0-3.0 (2.5-3.5 for mechanical heart valves). >5.0 = high bleeding risk.
Partial Thromboplastin Time (PTT)	25-35 seconds	>70 seconds	Monitor for bleeding. Used to monitor heparin therapy.

Arterial Blood Gases (ABGs)

Normal Values: pH: 7.35-7.45 | PaCO2: 35-45 mmHg | HCO3: 22-26 mEq/L | PaO2: 80-100 mmHg

Step-by-Step Interpretation:

1. Look at pH: <7.35 = acidosis, >7.45 = alkalosis
2. Look at PaCO2: >45 = respiratory acidosis, <35 = respiratory alkalosis
3. Look at HCO3: >26 = metabolic alkalosis, <22 = metabolic acidosis
4. Determine if compensation is present: If pH is abnormal but moving toward normal, it's compensated.

The 12 Lab Values That Show Up Most on NCLEX

If you only have time to memorize twelve, memorize these. Every one of them generates priority-action questions on the exam, and every one has a "hold the medication" or "call the provider" decision attached:

1. Potassium (K+) — 3.5–5.0 mEq/L. Hold digoxin if K+ < 3.5 (toxicity risk). Insulin pushes K+ into cells; replace K+ before/with insulin therapy in DKA.
2. Sodium (Na+) — 135–145. <120 = seizure risk; correct slowly to avoid central pontine myelinolysis.
3. Glucose (fasting) — 70–100. <70 = give 15 g fast carbs and recheck in 15 min (Rule of 15).
4. Calcium (total) — 8.5–10.5. Hypocalcemia → Trousseau and Chvostek signs, tetany, laryngospasm.
5. Magnesium — 1.5–2.5. Often co-replaced with K+ — hypomagnesemia makes K+ replacement ineffective.
6. Hemoglobin (Hgb) — 12–18 g/dL. <7 g/dL is a transfusion threshold for stable adults.
7. Hematocrit (Hct) — roughly 3× Hgb. Drops in acute bleed; rises in dehydration.
8. Platelets — 150,000–450,000. <50,000 = bleeding precautions; <20,000 = spontaneous bleed risk.
9. WBC — 4,000–11,000. <1,000 ANC = neutropenic precautions (low-bacteria diet, no fresh flowers, private room).
10. BUN — 7–20 mg/dL. Elevated in dehydration, GI bleed, kidney failure.
11. Creatinine — 0.6–1.2. The kidney number — rises with renal impairment; affects dose adjustments for nephrotoxic drugs.
12. INR — 0.9–1.1 untreated; 2–3 on warfarin (2.5–3.5 for mechanical valves). >5 = hold warfarin, consider vitamin K.

The "Hold the Drug" Lab Triggers Every NCLEX Tests

NCLEX loves a question structured as "the patient has these vitals/labs — should the nurse give this drug?" Memorize these specific hold thresholds:

• Digoxin — Hold if HR < 60 bpm OR digoxin level > 2.0 ng/mL OR K+ < 3.5 mEq/L (digoxin toxicity is potentiated by hypokalemia).
• Heparin (IV drip) — Adjust by aPTT; therapeutic range is typically 60–80 seconds (or 1.5–2.5× control). Hold and call provider if aPTT > 100.
• Warfarin — Hold if INR > 4 (or higher than the ordered range). Antidote: vitamin K.
• Beta-blockers (metoprolol, carvedilol) — Hold if HR < 60 bpm or SBP < 90 mmHg.
• ACE inhibitors / ARBs — Hold if SBP < 90 mmHg or potassium > 5.0 mEq/L (these drugs raise K+).
• Insulin — Always confirm K+ ≥ 3.5 mEq/L before initiating an insulin drip; insulin drives K+ intracellularly and can cause lethal arrhythmias if pre-existing hypokalemia.
• Furosemide (Lasix) — Reassess if K+ < 3.0 mEq/L; consider potassium replacement before next dose.
• Lithium — Therapeutic level 0.6–1.2 mEq/L. Hold and call provider for any level > 1.5 (toxicity).

The Critical-Value Phone Call

"Critical values" are lab results so dangerous they require immediate provider notification. The exact numbers vary by institution, but NCLEX expects you to recognize these as immediate-action triggers: K+ < 2.5 or > 6.5, Na+ < 120 or > 160, glucose < 50 or > 500, Ca < 6.0 or > 13.0, Hgb < 6 g/dL, platelets < 20,000 or > 1,000,000, INR > 5 on warfarin, and any troponin elevation. The exam will not always show "critical value" labeling — if a lab falls in one of these ranges and the patient is symptomatic, the priority answer is almost always "notify the provider," not "continue to monitor."

How to Memorize Without Burning Out

You will not memorize 30+ lab values by reading a sheet once. The pattern that works: encode in clinical context (pair every value with one nursing action — K+ 3.0 → hold digoxin; glucose 55 → give 15 g carbs); drill with practice questions until you've seen each in 30 different stems; use spaced repetition for the criticals (Day 1, Day 3, Day 7, Day 14 rewrites); and quiz with Nina — open the chat and type "quiz me on the 12 most-tested NCLEX lab values — give me the scenario, ask what's abnormal and what to do." See how to use Nina for more rapid-fire prompts.

How to Use This Cheat Sheet

Memorize the normal ranges and critical values for the most common labs. For more detailed information on specific lab values and their clinical significance, check out our comprehensive NCLEX Lab Values Guide.

The best way to learn lab values is to see them in context. On NCLEX PrePro, we incorporate lab values into our practice questions so you can learn how to interpret them in real clinical scenarios.

Ready to master lab values for the NCLEX? Take a free practice test and see how our questions can help you prepare. Or, get lifetime access to our entire platform for just $29.