Lab Values
Lab values appear across the NCLEX because safe nursing care depends on noticing when a result is dangerous and acting on it. You do not need to memorize every laboratory's reference range. You do need to know the common adult ranges, which abnormal results are urgent, and what the nurse does first. This topic covers the labs that most often drive nursing decisions.
Key facts
- Potassium 3.5–5.0 mEq/L: both low and high levels can cause dangerous dysrhythmias.
- Sodium 135–145 mEq/L: changes mainly show up as neurologic symptoms.
- Platelets 150,000–400,000/mm³: below 50,000 raises bleeding risk, and below 20,000 raises the risk of spontaneous bleeding.
- Arterial pH 7.35–7.45, PaCO2 35–45 mm Hg, HCO3 22–26 mEq/L.
- Serum creatinine about 0.6–1.2 mg/dL: a rising value suggests declining kidney function.
- Always check your facility's reference ranges. The NCLEX tests your judgment, not one lab's exact numbers.
Electrolytes that drive urgent action
Potassium is the electrolyte most likely to turn a routine shift into an emergency. Hyperkalemia can cause peaked T waves and life-threatening dysrhythmias. Hypokalemia causes muscle weakness, flattened T waves and a higher risk of digoxin toxicity. When a question shows an abnormal potassium level, look for the answer that protects the heart: cardiac monitoring, holding a medication that makes the problem worse, and notifying the provider.
Sodium problems usually show up as neurologic changes: confusion, headache, lethargy and, in severe cases, seizures. Correcting sodium too quickly can cause harm of its own, so NCLEX answers favor careful monitoring and prescribed replacement over aggressive fluid changes. Calcium and magnesium work together. Low levels of either can cause tetany, a positive Chvostek or Trousseau sign, and dysrhythmias.
- Hyperkalemia: monitor the heart and expect treatment to shift or remove potassium
- Hypokalemia: check digoxin, and never give IV potassium by push
- Hypocalcemia: have seizure precautions ready
Complete blood count
The CBC tells you about oxygen-carrying capacity, infection risk and bleeding risk. Hemoglobin of roughly 12–16 g/dL for adult females and 14–18 g/dL for adult males is a common teaching range. A low hemoglobin with tachycardia and dizziness points to poor tissue oxygenation, and the nurse's priority is safety and perfusion.
White blood cells normally run about 5,000–10,000/mm³. A very low count, especially a low absolute neutrophil count, calls for neutropenic precautions: meticulous hand hygiene, no fresh flowers or standing water, and prompt reporting of any fever. Low platelets call for bleeding precautions such as a soft toothbrush, an electric razor, no IM injections when avoidable, and watching for petechiae.
Coagulation and anticoagulants
Warfarin is monitored with the INR. A typical therapeutic goal is about 2–3 for many indications, and the provider sets the exact target. Unfractionated heparin is commonly monitored with the aPTT. On the NCLEX, a result far above the target alongside signs of bleeding means you hold the anticoagulant, assess the client, and notify the provider. Vitamin K is associated with warfarin reversal, and protamine sulfate with heparin reversal.
Questions often pair a coagulation value with a symptom such as black stools, gum bleeding or a sudden headache. The symptom tells you how urgent the situation is. A sudden severe headache in a client on anticoagulants suggests possible intracranial bleeding and calls for immediate escalation.
Kidney markers and ABGs
BUN (about 10–20 mg/dL) and creatinine help you judge kidney function. Creatinine is more specific to the kidneys. A rising creatinine should make you check medication doses, fluid balance and nephrotoxic drugs, and report falling urine output.
For ABGs, look at the pH first, then decide whether the PaCO2 (respiratory) or the HCO3 (metabolic) explains it. A client breathing slowly after opioids may show respiratory acidosis. A client with prolonged vomiting may show metabolic alkalosis. The nursing action follows the cause, for example supporting ventilation for respiratory acidosis.
Practice: 3 original questions
1. A client's potassium is 6.4 mEq/L. Which action should the nurse take first?
Answer and rationale
Correct: B. Severe hyperkalemia can cause fatal dysrhythmias, so cardiac monitoring comes first. The supplement and high-potassium foods would make things worse, and a urine collection does not address the immediate risk.
2. A client's platelet count is 18,000/mm³. Which prescription should the nurse question?
Answer and rationale
Correct: C. IM injections can cause bleeding and hematoma when platelets are critically low. The other options are standard bleeding precautions.
3. ABG results: pH 7.30, PaCO2 55 mm Hg, HCO3 24 mEq/L. How should the nurse interpret this?
Answer and rationale
Correct: B. The pH is low (acidosis) and the PaCO2 is high, while the HCO3 is normal. That pattern is uncompensated respiratory acidosis.
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