Lab Values You Must Know for the NCLEX, and What the Exam Does With Them

The exam tests the action, not the number

Students often memorise ranges and are then surprised when the exam does not ask for them. The NCLEX gives you a value and asks what you do: which finding to report, which intervention to take first, which medication to hold, which patient to see first.

This means recall is necessary but not sufficient. You need to know that a potassium of 6.8 is high, but the mark is awarded for knowing that it threatens cardiac conduction and warrants immediate action, and that you would expect to see peaked T waves.

Note also that reference ranges vary slightly between laboratories and textbooks. The exam uses values that are unambiguously abnormal for exactly this reason. You will not be asked to distinguish a sodium of 134 from one of 135. You will be asked about a sodium of 118.

The best academic structure does not decorate the clinical reasoning—it makes that reasoning visible.

Electrolytes

Sodium runs 135 to 145 mEq/L. Below 120 the risk is seizure and cerebral oedema; correct slowly, because rapid correction risks osmotic demyelination. Potassium runs 3.5 to 5.0 mEq/L and is the one to know cold: both ends are lethal. Above 6.0 expect peaked T waves and risk of ventricular fibrillation. Never give potassium by intravenous push under any circumstances.

Calcium runs 8.5 to 10.5 mg/dL. Low calcium produces neuromuscular irritability — Chvostek and Trousseau signs, tetany, laryngospasm. Magnesium runs 1.5 to 2.5 mEq/L; low magnesium causes tremor and dysrhythmia, and high magnesium causes loss of deep tendon reflexes, which is why reflexes are checked during magnesium sulphate infusions in pre-eclampsia.

Chloride runs 98 to 106 mEq/L and phosphorus 2.5 to 4.5 mg/dL. Phosphorus moves inversely to calcium, which is a relationship the exam likes to test in renal failure questions.

Why potassium dominates the exam

No single value appears more often than potassium, because it sits at the intersection of cardiac risk, renal function and common drugs. Loop and thiazide diuretics waste it. ACE inhibitors, angiotensin receptor blockers and potassium-sparing diuretics retain it. Insulin drives it into cells; so does a rising pH.

Low potassium presents as muscle weakness, cramping, constipation from reduced gut motility, and flattened T waves with U waves on the ECG. It also potentiates digoxin toxicity, which is why a question pairing a diuretic with digoxin is nearly always about potassium.

High potassium presents as weakness, paraesthesia and peaked T waves progressing to a widened QRS. Emergency management is calcium gluconate to stabilise the myocardium, then insulin with dextrose or a beta agonist to shift potassium into cells, then a binder or dialysis to remove it. Stabilise, shift, remove — in that order.

Complete blood count and coagulation

White cells run 5,000 to 10,000 per cubic millimetre. Below 5,000 the patient is at infection risk and may need neutropenic precautions; this is the value that drives chemotherapy questions. Haemoglobin runs roughly 12 to 16 g/dL in women and 14 to 18 in men, with haematocrit approximately three times the haemoglobin.

Platelets run 150,000 to 400,000 per cubic millimetre. Below 50,000 bleeding risk rises meaningfully and below 20,000 spontaneous bleeding becomes a real danger — implement bleeding precautions, avoid intramuscular injections, use a soft toothbrush and an electric razor.

For coagulation, know the pairings. Prothrombin time and INR monitor warfarin; therapeutic INR is generally 2 to 3, and the antidote is vitamin K. Activated partial thromboplastin time monitors heparin, with a therapeutic target of 1.5 to 2.5 times the control, and the antidote is protamine sulphate. Confusing these two pairings is one of the most common errors on the exam.

Renal function and arterial blood gases

Blood urea nitrogen runs 10 to 20 mg/dL and creatinine 0.6 to 1.2 mg/dL. Creatinine is the better marker of renal function because it is less affected by hydration and protein intake. A rising creatinine in a patient on a nephrotoxic drug such as vancomycin or gentamicin is a finding to report.

For arterial blood gases, pH runs 7.35 to 7.45, carbon dioxide 35 to 45 mmHg, and bicarbonate 22 to 26 mEq/L. Interpret in three steps. Is the pH acidotic or alkalotic? Does the carbon dioxide move in the opposite direction to the pH, indicating a respiratory cause? Does the bicarbonate move in the same direction as the pH, indicating a metabolic cause?

A pH of 7.28 with carbon dioxide of 52 and bicarbonate of 24 is respiratory acidosis: the pH is low, carbon dioxide is high and moving opposite to the pH, and bicarbonate is normal so there is no compensation yet. Think hypoventilation — opioid oversedation, chronic obstructive pulmonary disease, atelectasis.

Cardiac markers, glucose and liver studies

Troponin is the cardiac marker that matters. It begins rising three to four hours after myocardial injury, peaks around twelve hours and stays elevated for up to two weeks, which makes it useful for late presentations. Creatine kinase MB rises and falls faster and is largely of historical interest on the exam.

Fasting glucose runs 70 to 100 mg/dL. Haemoglobin A1c reflects roughly three months of control, with under 5.7 percent normal and under 7 percent a common target in diabetes. A patient whose fingerstick readings look acceptable but whose A1c is 9 percent is telling you something about the readings, not the A1c.

Albumin runs 3.5 to 5.0 g/dL and is a marker of both nutrition and hepatic synthesis. Ammonia rises in hepatic encephalopathy and is the value behind lactulose questions. Elevated ALT and AST indicate hepatocellular injury; ALT is the more liver-specific of the two.

Therapeutic drug levels worth memorising

Digoxin is therapeutic between 0.5 and 2.0 ng/mL. Above 2.0 suspect toxicity: nausea, vomiting, visual disturbance classically described as yellow-green halos, and bradycardia. Hypokalaemia potentiates digoxin toxicity, which is why the exam so often pairs a digoxin question with a diuretic.

Lithium is therapeutic between 0.6 and 1.2 mEq/L, with toxicity above 1.5. Lithium levels rise with dehydration and sodium depletion, so questions frequently involve vomiting, diarrhoea, diuretics or a low-sodium diet. Adequate fluid and consistent salt intake are the teaching points.

For patients on warfarin, remember that the value to monitor is the INR rather than the drug level itself, and that dietary vitamin K — leafy greens — should be kept consistent rather than eliminated. Consistency is the message, not avoidance.

How to memorise these without drowning

Do not try to learn every value in the textbook. The exam concentrates on perhaps twenty-five values, and the electrolytes plus the coagulation pairings account for a disproportionate share of them. Learn those cold before touching anything more obscure.

Learn each value together with one abnormal presentation and one nursing action. "Potassium 3.5 to 5.0; low causes weakness and flattened T waves; give oral or dilute IV replacement, never push" is one memory item, not three, and it is the form the exam actually asks about.

Then practise with questions rather than flashcards. Flashcards test recall; the exam tests application, and the gap between the two is exactly where marks are lost.