NCLEX lab values: the ranges to know cold
The exam does not test whether you can recite a range. It tests whether you notice the number that changes the plan: the potassium of 6.1 before the dose, the platelet count before the injection, the INR before the procedure. Here are the ranges most NCLEX reviews use, the critical values, and the action each one triggers.
How the exam uses lab values
Ranges vary a little between laboratories and textbooks, so NCLEX items avoid the edges: a value that matters in a stem is clearly abnormal, and when a precise cutoff matters, the item usually prints the reference range next to the result. When it does, use the one given. The values below are the common textbook ranges, in US units.
Electrolytes and chemistry
| Test | Normal (adult) | Critical / the action |
|---|---|---|
| Sodium | 135–145 mEq/L | Below 120 or above 160. Correct slowly (about 8–10 mEq/L per 24 h) to protect the brain |
| Potassium | 3.5–5.0 mEq/L | Below 2.5 or above about 6.0–6.5. Hold potassium-wasting or -sparing drugs as indicated; high K with ECG changes is an emergency. Never IV push |
| Calcium (total) | 9–10.5 mg/dL | Low: tetany, Chvostek's and Trousseau's signs, laryngospasm. High: weakness, stones, confusion |
| Magnesium | about 1.5–2.5 mEq/L | Low: tremor, hyperreflexia, torsades. High: lost reflexes, respiratory depression (check reflexes on a magnesium infusion) |
| Phosphorus | 2.5–4.5 mg/dL | Moves opposite to calcium; falls with refeeding |
| BUN | 10–20 mg/dL | Rises with dehydration, GI bleed and kidney dysfunction; high BUN with normal creatinine usually means dehydration |
| Creatinine | about 0.6–1.2 mg/dL | The specific kidney marker; doubling means GFR roughly halved. Check before contrast and nephrotoxic drugs |
| Fasting glucose | 70–99 mg/dL | Below 70 is hypoglycemia: 15 g carbohydrate if awake, IV dextrose or glucagon if not. Below 40 or above about 500 is critical |
| Hemoglobin A1c | below 5.7% | 6.5% or higher diagnoses diabetes; the treatment goal for most adults is under 7% |
Complete blood count
| Test | Normal (adult) | Critical / the action |
|---|---|---|
| Hemoglobin | men 14–18 g/dL, women 12–16 g/dL | Transfusion usually considered below 7–8; below about 6–7 is critical |
| Hematocrit | men 42–52%, women 37–47% | About three times the hemoglobin; rises with dehydration, falls with bleeding once fluids shift |
| WBC | 5,000–10,000/mm³ | Below about 2,000 or above about 30,000 is critical. Neutropenic precautions usually start below an ANC of about 1,000; under 500 is severe |
| Platelets | 150,000–400,000/mm³ | Bleeding precautions below about 50,000; spontaneous bleeding risk below 20,000; a critical value around 10,000–20,000 |
Coagulation
| Test | Normal | On therapy |
|---|---|---|
| PT / INR | PT about 11–13.5 s; INR 0.8–1.2 | Warfarin: INR 2–3 for most uses (about 2.5–3.5 with a mitral or high-risk valve). Above 5 is critical: hold warfarin, vitamin K per order |
| aPTT | about 30–40 s | Heparin infusion: 1.5–2.5 times control (roughly 46–70 s). Above about 70 s is critical: stop or slow the infusion per protocol, protamine for severe bleeding |
| Anti-Xa (heparin) | — | About 0.3–0.7 units/mL for unfractionated heparin where used instead of the aPTT |
| Fibrinogen / D-dimer | fibrinogen about 200–400 mg/dL; D-dimer under about 0.5 mcg/mL | Falling fibrinogen with a rising D-dimer in a bleeding client suggests DIC |
Arterial blood gases
| Test | Normal | Read it as |
|---|---|---|
| pH | 7.35–7.45 | Below 7.35 acidosis, above 7.45 alkalosis; below about 7.2 or above 7.6 is critical |
| PaCO₂ | 35–45 mmHg | The respiratory side: high with hypoventilation (COPD, opioids), low with hyperventilation |
| HCO₃⁻ | 22–26 mEq/L | The metabolic side: low in DKA and diarrhea, high with vomiting |
| PaO₂ | 80–100 mmHg | Below 80 hypoxemia; below 60 usually needs more oxygen or support; below about 40 critical |
| SaO₂ | 95–100% | Target 88–92% in COPD clients who retain CO₂ |
ROME: Respiratory Opposite (pH and PaCO₂ move in opposite directions), Metabolic Equal (pH and HCO₃⁻ move together). ABGs in Three Steps: ROME does the three-step read.
Liver, cardiac and drug levels
| Test | Range | Notes |
|---|---|---|
| Albumin | 3.5–5 g/dL | Low albumin drags total calcium down; check ionized calcium |
| Ammonia | about 15–45 mcg/dL | Climbs in liver failure; confusion and asterixis; lactulose |
| ALT / AST | ALT about 7–40 U/L; AST about 10–40 U/L | ALT is the more liver-specific one; both rise with acetaminophen toxicity |
| Bilirubin (total) | about 0.1–1.2 mg/dL | Jaundice visible above about 2–3 |
| Troponin | essentially undetectable | The specific marker of heart muscle injury, drawn in series |
| BNP | under about 100 pg/mL | Above 100 supports heart failure |
| Lactate | under 2 mmol/L | 4 or more in sepsis triggers the fluid bolus |
| Digoxin | 0.5–2.0 ng/mL | Toxicity: nausea, bradycardia, visual halos; low potassium makes it worse |
| Lithium | 0.6–1.2 mEq/L | Above about 1.5: coarse tremor, vomiting, ataxia; dehydration raises it |
| Phenytoin | 10–20 mcg/mL | Above 20: nystagmus, ataxia, slurred speech |
| Vancomycin | trough 10–20 mcg/mL (15–20 for serious MRSA), or AUC 400–600 | Many pharmacies now dose to AUC rather than trough |
| Magnesium sulfate (preeclampsia) | about 4–7 mEq/L | Lost deep tendon reflexes come first, then respirations under 12 and urine under 30 mL/h; calcium gluconate is the antidote |
How to memorize them
Learn the action with the number, not the number alone. Potassium is not "3.5 to 5"; it is "below 3.5, flat T waves, hold the digoxin question; above 5, peaked T waves, no IV push, call". Make one card per value with the critical cutoff on the back and the nurse's move under it. The app's lab deck has 73 values in that shape, and The Lab Values You Must Recite in Your Sleep gives the ten you must recite in your sleep.
✅ Test yourself: 4 questions
Four lab-value items. In each, find the number that changes what the nurse does next. Every client is fictional; exam practice only, not medical advice.
Question 1 of 4
A nurse reviews the platelet counts of four clients. None of the clients is actively bleeding. For which client should the nurse anticipate a prescription for a platelet transfusion?
- Acute leukemia on induction chemotherapy: platelets 8,000/mm³
- Immune thrombocytopenia: platelets 16,000/mm³ and a few leg petechiae
- Heparin-induced thrombocytopenia: platelets 54,000/mm³ and a new DVT
- Platelets 36,000/mm³ before a central venous catheter is inserted
Show the answer and rationale
Answer: A
When chemotherapy stops the marrow from making platelets, a nonbleeding client receives a prophylactic transfusion once the count falls below 10,000/mm³ (AABB 2025), so the client with leukemia at 8,000 is the one to anticipate. In immune thrombocytopenia, antibodies quickly destroy transfused platelets, so treatment is a corticosteroid or immune globulin, with platelets saved for serious bleeding. In heparin-induced thrombocytopenia, platelet transfusion is generally avoided because it has been linked to more arterial clots; heparin is stopped and a nonheparin anticoagulant started. Central line insertion at a compressible site calls for platelets only below about 10,000/mm³.
Question 2 of 4
A client with type 2 diabetes who takes metformin 1,000 mg twice daily has an eGFR of 42 mL/min/1.73 m² and is scheduled for a coronary angiogram with intra-arterial iodinated contrast tomorrow. Which instruction about the metformin is accurate?
- Stop it 48 hours before the angiogram and restart it the evening after the procedure ends
- Keep taking it on the usual schedule, because any eGFR above 30 mL/min/1.73 m² allows its use
- Stop it at the time of the angiogram and restart it after a kidney recheck 48 hours later
- Stop it permanently and change to another drug, because an eGFR of 42 is too low for its use
Show the answer and rationale
Answer: C
The metformin label directs that it be stopped at the time of, or before, an iodinated contrast procedure in clients with an eGFR of 30 to 60 mL/min/1.73 m² or who receive intra-arterial contrast, and restarted only if the eGFR rechecked 48 hours later is stable; if the contrast injures the kidneys, metformin can accumulate and cause lactic acidosis. Continuing as usual is the runner-up because an eGFR above 30 permits ongoing daily use, but this client meets both conditions for holding it. Stopping 48 hours ahead reverses the timing, and an eGFR of 42 does not require stopping for good; metformin is contraindicated only below 30.
Question 3 of 4
A 24-year-old client with a seizure disorder is starting valproic acid. Which statement by the client indicates understanding of the teaching?
- 'I'll use reliable birth control and plan any pregnancy with my provider.'
- 'If I find out I'm pregnant, I'll stop the medicine right away.'
- 'I'll take folic acid every day, so I won't need birth control while I'm on it.'
- 'If my blood levels stay in range, it's safe for me to become pregnant on it.'
Show the answer and rationale
Answer: A
Valproate carries a boxed warning for major birth defects, including neural tube defects, and lower IQ after prenatal exposure, so effective contraception and planning any pregnancy with the provider, who may switch drugs and add folic acid, is correct. The runner-up is daily folic acid: it lowers neural tube defect risk but does not make valproate safe in pregnancy or replace contraception. Stopping abruptly can trigger status epilepticus, so a pregnant client calls the provider rather than quitting, and an in-range blood level does not remove the birth-defect risk.
Question 4 of 4
The telemetry technician calls the nurse about four clients whose morning potassium levels are elevated. Which client should the nurse see first?
- Potassium 5.8 mEq/L with a normal ECG, in a client due for dialysis this afternoon
- Potassium 6.2 mEq/L with a QRS widened from 0.08 to 0.14 seconds, after a crush injury
- Potassium 6.6 mEq/L with tall, peaked T waves, in a client with chronic kidney disease
- Potassium 5.6 mEq/L before an insulin infusion starts, in a client with DKA
Show the answer and rationale
Answer: B
A widening QRS is a late sign of hyperkalemia: the ECG moves from peaked T waves to a longer PR interval and flattened P waves, then a wide QRS that can merge into a sine wave and ventricular fibrillation. The crush-injury client is closest to arrest despite the lower number, and damaged muscle keeps releasing potassium. The client with 6.6 mEq/L and peaked T waves is the tempting choice and needs prompt treatment too, but peaked T waves are the earliest change. A level of 5.6 mEq/L is expected in DKA before insulin, and 5.8 mEq/L with a normal ECG can wait for scheduled dialysis.
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73 lab-value flashcards with normal and critical ranges, plus practice items scored by area. No ads, no trial, no card.
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❓ Frequently asked
Which lab values are most tested on the NCLEX?
Potassium, sodium, glucose, hemoglobin, platelets, WBC/ANC, INR and aPTT on anticoagulants, creatinine and BUN, ABGs, and the therapeutic levels of digoxin, lithium and phenytoin.
Does the NCLEX give you the normal ranges?
Often, in Next Gen case studies the client record prints a reference range beside the result. Stand-alone items may not, so know the common ranges and the critical cutoffs cold.
What is a critical lab value?
A result far enough outside normal that it threatens life without prompt action, such as potassium above about 6.0–6.5, glucose below 40, platelets below 20,000 or an INR above 5. Laboratories call these in; the nurse acts and notifies.
What is the normal potassium range?
3.5 to 5.0 mEq/L (some labs 3.5 to 5.2). Below 2.5 or above about 6.0 to 6.5 is critical.
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Sources
- OpenStax, Clinical Nursing Skills, Appendix A: Adult Lab Values
- Cleveland Clinic Laboratories, Critical and Urgent Values
- MedlinePlus Medical Encyclopedia (NIH), individual lab tests
- NCSBN, NCLEX-RN Test Plan effective April 2026 (categories, 85–150 items, case studies, scoring methods, how the exam ends)
Accessed October 2026. Yearly figures name their year; NCSBN publishes a new statistics brief each June and a new test plan every three years (next: April 2029). Exam preparation only, never medical advice. NCLEX® is a registered trademark of NCSBN, which does not endorse this site.
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