NCLEX lab values: the 24 worth memorizing
Written and reviewed by Dana Whitfield, RN, MSN · 8 min read · Updated August 2026
Short answer
You need about two dozen lab values cold, not the whole panel. Potassium 3.5–5.0, sodium 135–145, and the therapeutic drug ranges account for a disproportionate share of items, because the exam does not test recall of a number — it tests what you do about it. Memorize the value with its nursing action attached or the number is worthless.
Electrolytes: the six that carry the questions
Potassium 3.5–5.0 mEq/L, sodium 135–145 mEq/L, calcium 9.0–10.5 mg/dL, magnesium 1.3–2.1 mEq/L, phosphorus 3.0–4.5 mg/dL, chloride 98–106 mEq/L.
Potassium is the one to know perfectly. It appears constantly because it kills, and the exam wants the action: below 3.5 with a client on digoxin means toxicity risk; above 5.0 means peaked T waves and hold the potassium-sparing diuretic; never IV push, always diluted and on a pump. Magnesium is potassium's shadow — a potassium that will not correct is usually a magnesium problem, and that inference is a classic item.
Renal, glucose, and the numbers that trigger a hold
BUN 10–20 mg/dL, creatinine 0.6–1.2 mg/dL, GFR above 90, fasting glucose 70–110 mg/dL, HbA1c below 5.7%.
Creatinine is the one that reads as boring and functions as a trap. A rise from 0.9 to 1.8 is still inside 'not that high' for anyone reading the number alone, and it is a doubling — the trigger to hold the nephrotoxic drug, hold the contrast, and call. The exam repeatedly rewards candidates who read trend rather than threshold.
Haematology and coagulation
Hemoglobin 12–18 g/dL, hematocrit 36–52%, platelets 150,000–400,000, WBC 5,000–10,000. INR 0.8–1.1 normal and 2.0–3.0 therapeutic on warfarin; aPTT 30–40 seconds normal and 46–70 therapeutic on heparin; PT 11–12.5 seconds.
Learn the therapeutic windows as pairs with their antidote: warfarin with vitamin K, heparin with protamine sulfate. A large share of coagulation items are really antidote items wearing a lab coat.
Therapeutic drug levels and ABGs
Digoxin 0.5–2.0 ng/mL, lithium 0.6–1.2 mEq/L, phenytoin 10–20 mcg/mL, theophylline 10–20 mcg/mL. All four have narrow windows, all four have a signature toxicity picture, and all four turn up as 'which finding do you report' items.
ABGs: pH 7.35–7.45, PaCO2 35–45 mm Hg, HCO3 22–26 mEq/L, PaO2 80–100 mm Hg. Do not memorize interpretation tables — run ROME on the spot. Respiratory Opposite, Metabolic Equal: pH down and CO2 up is respiratory acidosis; pH down and bicarb down is metabolic acidosis. Two comparisons, no table.
How to actually retain them
Do not study lab values as a list. Study them inside questions, where the number arrives attached to a client who is on a drug and has a symptom, because that is the only form in which the exam ever presents one.
The retrieval you need on the day is 'potassium 2.9 in a client on furosemide and digoxin → report before the next dose', and that path never gets built by reading a table.
Whatever you take from this, the next step is the same: answer questions and read the rationales. Our dosage calculation and lab values practice questions are the closest set to what this guide covers, there are ten more on the practice questions hub, and the pricing page spells out what the free tier includes.
Common questions
How many lab values do I need to know for the NCLEX?
Around 24 carry nearly all the items: six electrolytes, the renal and glucose values, the haematology and coagulation set, four therapeutic drug levels, and the four ABG values. Each needs its nursing action attached, not just its range.
What is the most important lab value on the NCLEX?
Potassium, 3.5–5.0 mEq/L. It appears more than any other because abnormal potassium is immediately dangerous, interacts with digoxin and diuretics, and drives a clear nursing action.
Are lab value ranges given to you on the NCLEX?
No. Some Next Generation case studies display a chart with reference ranges alongside the results, but standalone items routinely give a bare number and expect you to know whether it is a problem.