RN BasicsClinical review
NCLEX Lab Values to Know: Ranges and Critical Values
The NCLEX lab values to know are a short core set: sodium, potassium, calcium, magnesium, glucose, creatinine, the blood count and the clotting tests, plus the critical values that turn a lab result into an action. The exam prints the normal range with every numeric lab value, so the skill being tested is not reciting a range but knowing how far off a value is and what it can do to the client now.
This post gives typical adult ranges from the National Library of Medicine, explains why your textbook and the exam may show slightly different numbers, lists the critical limits most US hospitals use, and walks through two lab questions from the RN Basics bank.
NCLEX lab values to know: the core ranges
These are typical adult reference ranges in US units. Learn the shape of each one (roughly where normal starts and ends) and the direction that is dangerous. Ranges differ slightly from lab to lab because of different methods and samples.
| Test | Typical adult range | Why it matters on the exam |
|---|---|---|
| Sodium | 135 to 145 mEq/L | Low or high: confusion, seizures |
| Potassium | 3.7 to 5.2 mEq/L (many texts 3.5 to 5.0) | Low or high: dysrhythmias |
| Calcium (total) | 8.8 to 10.3 mg/dL | High: raises digoxin toxicity risk |
| Magnesium | 1.7 to 2.2 mg/dL | Low: raises digoxin toxicity risk |
| Glucose (fasting) | 70 to 99 mg/dL | 100 to 125 impaired; 126 or more suggests diabetes |
| Creatinine | Men 0.7 to 1.3; women 0.5 to 0.95 mg/dL | Rising: kidney injury, drug dosing |
| WBC | 4,500 to 11,000/mcL | Low: infection risk; high: infection, inflammation |
| Hemoglobin | Men 13.8 to 17.2; women 12.1 to 15.1 g/dL | Falling: bleeding; trend it |
| Platelets | 150,000 to 400,000/mcL | Below 50,000: higher bleeding risk |
| PT / INR | 11 to 13.5 s / 0.8 to 1.1 | On warfarin the INR goal is usually 2.0 to 3.0 |
Why lab ranges vary, and which one to use
Potassium is the classic example. The National Library of Medicine lists 3.7 to 5.2 mEq/L; many nursing references teach 3.5 to 5.0; and a 2025 consensus panel notes that the definition of hyperkalemia ranges from above 4.5 to above 5.5 depending on the guideline. Hemoglobin ranges differ by source and by sex.
- On the exam: use the range printed in the item. If a value sits just outside it, ask whether the client has symptoms or a reason for the change.
- In study: learn one consistent set, such as the one on the Lab Values to Know cheat sheet, rather than switching between books.
- In practice: use your own lab's range and its critical-value list.
Critical values that change the nursing action
A critical value is a result so far from normal that it may threaten life unless someone acts. Each hospital publishes its own list. A 2025 review of 50 critical-value lists from major US hospitals gives a fair picture of where most of them draw the line.
| Test | Typical low critical | Typical high critical | First nursing concern |
|---|---|---|---|
| Potassium | 2.8 mEq/L | 6.0 mEq/L | Dysrhythmia: cardiac monitor or 12-lead ECG |
| Sodium | 120 mEq/L | 160 mEq/L | Neurologic change, seizures |
| Glucose | 50 mg/dL | 500 mg/dL | Low: treat now; high: DKA or HHS |
| Calcium (total) | 6.0 mg/dL | 13.0 mg/dL | Neuromuscular and cardiac effects |
| Hemoglobin | 6.0 g/dL | 20.0 g/dL | Bleeding, perfusion, transfusion |
| Platelets | 20,000/mcL | 1,000,000/mcL | Bleeding precautions |
| INR | (none) | 5.0 | Bleeding; hold anticoagulant and report |
| WBC | 1,400/mcL | 50,000/mcL | Low: infection precautions |
| Arterial pH | 7.20 | 7.60 | Find the cause: respiratory or metabolic |
When an item gives you a critical value, the right answer usually does three things in order: assess the client for the effect that value causes, take the protective step that is within nursing scope (cardiac monitoring for potassium, seizure precautions for sodium, bleeding precautions for platelets, holding the related drug), and report the result promptly to the provider.
Lab values that work together
- Potassium and digoxin: low potassium, low magnesium or high calcium raise the risk of digoxin toxicity.
- Potassium and loop diuretics: furosemide can cause hypokalemia, especially with brisk diuresis.
- Insulin and potassium: insulin moves potassium into cells, which is why it is used for high potassium and why potassium is watched during an insulin infusion.
- Platelets and bleeding: below about 50,000 the bleeding risk rises, and most hospitals treat a count near 20,000 as critical.
Pairings like these are where the exam goes beyond memorization. The Electrolytes cheat sheet and the High-Alert Meds and Insulin cheat sheet cover the drug and electrolyte links in more detail.
Two lab questions, worked through
The nurse reviews the morning laboratory results for a client receiving furosemide and digoxin. Which value is most important to report to the provider?
- Fasting glucose 112 mg/dL
- Blood urea nitrogen 18 mg/dL
- Sodium 138 mEq/L
- Potassium 2.9 mEq/L
Show the answer and rationale
Answer: D
- A. Mild elevation is not urgent.
- B. This is within the normal range.
- C. This is a normal sodium.
- D. Correct. Hypokalemia from the diuretic increases the risk of digoxin toxicity and dysrhythmias.
Normal potassium is 3.5 to 5.0 mEq/L. Loop diuretics waste potassium, and low potassium sensitizes the heart to digoxin. The nurse reports the level, holds digoxin if toxicity signs are present, anticipates potassium replacement and cardiac monitoring, and teaches high-potassium foods.
Bottom line: Loop diuretics waste potassium, and hypokalemia sensitizes the heart to digoxin, so a low potassium in a client taking both is reported promptly.
Sources: Ignatavicius, Med-Surg Nursing, 11th ed.; Lehne's Pharmacology for Nursing, 12th ed.
A client with normal kidney function is admitted for cellulitis. Yesterday’s potassium was 4.0 mEq/L. This morning’s result is 6.2 mEq/L, and the laboratory flags the specimen as hemolyzed after a difficult draw. The client has no symptoms, and the cardiac monitor shows no new changes. Which action should the nurse take?
- Recheck the level with tomorrow’s routine labs
- Give IV calcium gluconate and insulin with dextrose now
- Start a low-potassium diet and repeat in 24 hours
- Get a 12-lead ECG and request a prompt repeat sample
Show the answer and rationale
Answer: D
- A. Assuming hemolysis is reasonable, but waiting a day leaves a possible true hyperkalemia unconfirmed and unmonitored.
- B. Tempting because 6.2 mEq/L is a treatable level, but treating a likely false result exposes the client to hypoglycemia and other harm before it is confirmed.
- C. A diet change does not address an acute value of 6.2 mEq/L and still delays the confirmation the client needs today.
- D. Correct. The result is probably an artifact, but a normal rhythm strip does not exclude true hyperkalemia, so an ECG and a quick repeat draw settle it.
Pseudohyperkalemia is a laboratory artifact: potassium leaks from cells in the tube when the sample is hemolyzed after a difficult draw, delayed in transit, taken from a client with a very high platelet count, contaminated with EDTA from a blood count tube drawn first, or taken from the arm with a potassium infusion. A sudden jump with normal kidney function and no ECG change suggests it, but a normal tracing does not protect against arrhythmia if the value is real. So the nurse confirms with a 12-lead ECG and a careful repeat draw from a large vein, and treats only a confirmed result, because insulin and dextrose given for a false value can cause hypoglycemia.
Bottom line: Suspect pseudohyperkalemia when a hemolyzed sample shows a sudden potassium rise with normal kidney function, but confirm with an ECG and careful repeat draw before treating.
Sources: UKKA Hyperkalaemia Guideline (2026)
Question 1: furosemide, digoxin and four results
Check each value against its range first. Sodium 138 and BUN 18 are normal, a fasting glucose of 112 is only mildly high, and none of them threatens the heart today. Potassium 2.9 is clearly low. Then add the drugs: furosemide is what lowered it, and low potassium is one of the conditions the digoxin label names as predisposing to toxicity. A low potassium in a client on both drugs is a dysrhythmia risk, so it is the value to report.
Question 2: a high potassium that may not be real
A potassium of 6.2 would normally call for urgent treatment, but several clues point to a false result: normal kidney function, a normal value yesterday, no symptoms, and a specimen the lab flagged as hemolyzed after a difficult draw. Hemolysis is the most common cause of spurious hyperkalemia, and the consensus advice is to recheck a hemolyzed sample.
Treating now with calcium and insulin risks giving insulin and dextrose for a number that is not real. Waiting until tomorrow ignores the chance that it is real. The safe middle path is a 12-lead ECG to look for the effects of a true high potassium, plus a prompt repeat sample.
How to study lab values for the NCLEX
- Learn the ten core ranges above until you can sketch them from memory, then stop polishing them.
- For each one, learn the symptom of too low and too high, and the drug that most often pushes it there.
- Learn the critical limits as approximate thresholds, not exact numbers.
- Practice questions that give four abnormal values and ask which comes first; the answer is the one that can harm soonest.
- Pair lab review with your pharmacology review, since most lab questions are really about a drug.
Frequently asked questions
Does the NCLEX give you normal lab values?
Yes. NCSBN says items that include a numeric lab value also show the normal reference range, so you are judging the value rather than recalling the range.
What lab values should I memorize for the NCLEX?
Sodium, potassium, calcium, magnesium, glucose, creatinine, WBC, hemoglobin, platelets and PT/INR cover most items. Know which direction is dangerous and the usual critical thresholds.
What potassium level is critical?
Most US hospital lists use about 6.0 mEq/L as the high critical limit and about 2.8 as the low one, though each lab sets its own.
Why do lab ranges differ between books?
Each laboratory sets ranges for its own methods and samples, and books copy different sources. Use the range printed in the exam item.
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Related cheat sheets
- Lab Values to KnowNormal ranges in US units, therapeutic drug levels, and acting on a critical result
- Electrolyte ImbalancesNa, K, Ca, Mg and phos: normals, ECG clues, safe replacement, hyperkalemia steps
- ABG Interpretation in 6 StepspH, PaCO₂, HCO₃⁻, compensation, anion gap and oxygenation, with worked examples
- High-Alert Meds and InsulinISMP list, insulin timing, rule of 15, anticoagulants and reversal, digoxin, IV potassium, naloxone, safe orders and the rights
- NCLEX-RN Exam Guide2026 test plan: format, CAT pass rules, content weights, eligibility, an 8-week plan and test-day tips
Sources
- NCSBN Help Center. Do items provide candidates with normal reference ranges? Link
- NCSBN Help Center. Which units of measurement will be used with lab values on the NCLEX exam? Link
- MedlinePlus. Sodium blood test. Link
- MedlinePlus. Potassium blood test. Link
- MedlinePlus. Calcium blood test. Link
- MedlinePlus. Magnesium blood test. Link
- MedlinePlus. Blood sugar test. Link
- MedlinePlus. Creatinine blood test. Link
- MedlinePlus. WBC count. Link
- MedlinePlus. Hemoglobin. Link
- MedlinePlus. Platelet count. Link
- MedlinePlus. Prothrombin time (PT). Link
- Shah A, et al. Visualization of critical limits and critical values facilitates interpretation. Diagnostics (Basel). 2025. Link
- EMCREG-International multidisciplinary consensus panel on management of hyperkalemia in chronic kidney disease and heart failure. Cardiorenal Med. 2025. Link
- Furosemide injection, prescribing information (DailyMed). Link
- Digoxin tablets, prescribing information (DailyMed). Link
- Eli Lilly and Company. HUMULIN R (insulin human) injection, prescribing information (DailyMed). Link