Ten Lab Values to Know Cold Before the NCLEX
May 4, 2026 · NursingFloor
There are hundreds of lab values. The NCLEX hammers about ten of them. Here are the ones to commit to memory, with the clinical reasoning behind each.
You don't need to memorize every lab value in the textbook. The NCLEX tests a core set, and clinical practice requires you to know them cold. Here are the ten that show up over and over. Verify exact ranges against your program's reference and current authoritative sources. Hospitals use slightly different cutoffs.
**1. Potassium (K+): 3.5 to 5.0 mEq/L**
Why it matters: cardiac arrhythmias. Low potassium causes muscle weakness, fatigue, and serious cardiac changes. High potassium can stop the heart. NCLEX loves to test the relationship between potassium and digoxin, diuretics, and ACE inhibitors. Know that you don't push IV potassium fast, ever. Always diluted, always slow.
**2. Sodium (Na+): 135 to 145 mEq/L**
Why it matters: neuro changes. Low sodium causes seizures and confusion. High sodium causes thirst and dehydration. The classic NCLEX question links sodium changes to mental status changes.
**3. Hemoglobin: 12 to 18 g/dL (varies by sex)**
Why it matters: oxygen-carrying capacity. Low hemoglobin means anemia, fatigue, pallor, and tissue hypoxia. The NCLEX will tie low hemoglobin to symptoms (tachycardia, shortness of breath) and ask what to do first. Trends matter more than single values.
**4. Hematocrit: 36 to 52% (varies by sex)**
Why it matters: percent of blood that's red blood cells. Roughly three times the hemoglobin. Low hematocrit signals anemia or bleeding. High hematocrit can signal dehydration or polycythemia.
**5. Platelets: 150,000 to 400,000/mm³**
Why it matters: bleeding risk. Under 50,000, the patient is at high bleeding risk. Under 20,000, spontaneous bleeding. NCLEX questions: when to hold anticoagulants, when to call about a bleed, when bleeding precautions kick in.
**6. WBC: 5,000 to 10,000/mm³**
Why it matters: infection. High WBC suggests infection or inflammation. Low WBC (especially neutropenia under 1,000) means the patient is immunocompromised and needs neutropenic precautions. Know the precautions.
**7. INR: 0.8 to 1.1 (therapeutic on warfarin: 2 to 3)**
Why it matters: bleeding risk on anticoagulation. Above 3 (or above 3.5 for some indications) increases bleeding. The NCLEX classic: what to do when a patient on warfarin has an INR of 8. The answer involves vitamin K and holding the warfarin.
**8. Creatinine: 0.6 to 1.2 mg/dL**
Why it matters: kidney function. Rising creatinine means failing kidneys. This affects which medications are safe (metformin, NSAIDs, contrast dye) and at what doses. NCLEX likes to ask about contraindications when kidney function is reduced.
**9. BUN: 7 to 20 mg/dL**
Why it matters: kidney function and hydration status. Elevated BUN with normal creatinine often means dehydration. Both elevated means renal dysfunction.
**10. Glucose: 70 to 110 mg/dL fasting**
Why it matters: diabetic emergencies. Under 70 is hypoglycemia (give 15g fast-acting carbs, recheck in 15 minutes). Over 250 in the diabetic patient signals hyperglycemia and possible DKA. NCLEX loves the 15-15 rule.
**How to learn these**
Don't memorize ranges in isolation. Pair each lab with one clinical pattern: hypokalemia + muscle weakness, low platelets + bleeding precautions, high BUN + dehydration. The NCLEX tests the connection, not the number. The number is just the trigger.
Always verify against current authoritative sources and your facility's reference ranges before relying on these in practice.
Found this useful? Share it with someone who needs it.