Complete Blood Count (CBC)
| Lab | Normal Range | Critical Low | Critical High | Nursing Implication |
|---|---|---|---|---|
| WBC | 4,500–11,000/mm³ | <2,000 | >30,000 | Low = infection risk (neutropenic precautions); High = infection or leukemia |
| RBC (Male) | 4.5–5.5 million/mm³ | <3.0 | >6.5 | Low = anemia; High = polycythemia |
| RBC (Female) | 4.0–5.0 million/mm³ | <3.0 | >6.5 | Low = anemia; High = polycythemia |
| Hemoglobin (Male) | 14–18 g/dL | <7 | >20 | Low = fatigue, tachycardia; transfuse if <7 or symptomatic |
| Hemoglobin (Female) | 12–16 g/dL | <7 | >20 | Low = fatigue, tachycardia; transfuse if <7 or symptomatic |
| Hematocrit (Male) | 40–54% | <25% | >60% | Low = bleeding/anemia; High = dehydration or polycythemia |
| Hematocrit (Female) | 36–48% | <25% | >60% | Low = bleeding/anemia; High = dehydration or polycythemia |
| Platelets | 150,000–400,000/mm³ | <50,000 | >1,000,000 | Low = bleeding precautions; <20,000 = spontaneous bleeding risk |
Basic Metabolic Panel (BMP)
| Lab | Normal Range | Critical Low | Critical High | Nursing Implication |
|---|---|---|---|---|
| Sodium (Na⁺) | 136–145 mEq/L | <120 | >160 | Low = confusion, seizures; High = thirst, dry mucous membranes |
| Potassium (K⁺) | 3.5–5.0 mEq/L | <3.0 | >6.0 | CRITICAL: Cardiac dysrhythmias at both extremes. Check ECG. |
| Chloride (Cl⁻) | 98–106 mEq/L | <80 | >115 | Follows sodium; Low in vomiting (metabolic alkalosis) |
| CO₂ / Bicarb (HCO₃) | 22–26 mEq/L | <15 | >40 | Low = metabolic acidosis; High = metabolic alkalosis |
| BUN | 10–20 mg/dL | <5 | >100 | High = dehydration or renal failure; check with creatinine |
| Creatinine | 0.6–1.2 mg/dL | — | >4.0 | Best indicator of renal function. Rising = kidney injury |
| Glucose (Fasting) | 70–100 mg/dL | <40 | >400 | Low = give 15g carbs (15-15 rule); High = assess for DKA/HHS |
| Calcium (Ca²⁺) | 8.5–10.5 mg/dL | <6.0 | >13.0 | Low = Trousseau/Chvostek signs, tetany; High = cardiac arrest risk |
| Magnesium (Mg²⁺) | 1.5–2.5 mEq/L | <1.0 | >4.0 | Low = tremors, seizures; High = hyporeflexia, respiratory depression |
| Phosphorus | 2.5–4.5 mg/dL | <1.0 | >7.0 | Inverse relationship with calcium. Low = muscle weakness |
Coagulation Studies
| Lab | Normal Range | Critical Low | Critical High | Nursing Implication |
|---|---|---|---|---|
| PT | 11–13.5 seconds | — | >20 | Monitors warfarin therapy. Elevated = bleeding risk |
| INR | 0.8–1.1 (2.0–3.0 on warfarin) | — | >4.5 | Warfarin target: 2.0–3.0 (mechanical valve: 2.5–3.5) |
| aPTT | 25–35 seconds | — | >70 | Monitors heparin therapy. Target: 1.5–2.5× control |
| D-Dimer | <500 ng/mL | — | >500 | Elevated = clot breakdown (DVT, PE, DIC). Sensitive not specific |
| Fibrinogen | 200–400 mg/dL | <100 | — | Low in DIC. Critically low = massive hemorrhage risk |
Cardiac Markers
| Lab | Normal Range | Critical Low | Critical High | Nursing Implication |
|---|---|---|---|---|
| Troponin I | <0.04 ng/mL | — | >0.4 | GOLD STANDARD for MI. Rises 3–6h, peaks 12–24h |
| BNP | <100 pg/mL | — | >900 | Heart failure marker. Higher = worse HF severity |
| CK-MB | <5 ng/mL | — | >10 | Cardiac-specific CK isoenzyme. Rises 4–8h after MI |
| CK (Total) | 30–170 U/L | — | >10,000 | Very high = rhabdomyolysis. Hydrate aggressively |
Liver Function Tests (LFTs)
| Lab | Normal Range | Critical Low | Critical High | Nursing Implication |
|---|---|---|---|---|
| AST (SGOT) | 10–40 U/L | — | >1,000 | Liver AND heart/muscle. Very high = acute hepatitis |
| ALT (SGPT) | 7–56 U/L | — | >1,000 | Most specific liver enzyme |
| Albumin | 3.5–5.0 g/dL | <2.0 | — | Low = malnutrition, liver disease. Edema risk |
| Bilirubin (Total) | 0.1–1.2 mg/dL | — | >12 | Elevated = jaundice |
| Ammonia | 15–45 mcg/dL | — | >100 | High = hepatic encephalopathy. Give lactulose |
Thyroid Function
| Lab | Normal Range | Critical Low | Critical High | Nursing Implication |
|---|---|---|---|---|
| TSH | 0.5–4.0 mIU/L | <0.01 | >10 | HIGH TSH = hypothyroid; LOW TSH = hyperthyroid |
| Free T4 | 0.8–1.8 ng/dL | <0.4 | >4.0 | Low = hypothyroid; High = hyperthyroid |
| Free T3 | 2.3–4.2 pg/mL | <1.0 | >8.0 | Most active thyroid hormone |
Frequently Asked Questions
What are the most important lab values to memorize for the NCLEX?
Focus on potassium (3.5–5.0 mEq/L), sodium (136–145 mEq/L), calcium (8.5–10.5 mg/dL), BUN (10–20 mg/dL), creatinine (0.6–1.2 mg/dL), glucose (70–100 mg/dL fasting), hemoglobin (12–18 g/dL), platelets (150,000–400,000/mm³), INR (therapeutic on warfarin 2.0–3.0), and troponin (<0.04 ng/mL). These appear most frequently and drive the highest-priority nursing actions.
What is a critical potassium level?
Critical potassium is <3.0 mEq/L (hypokalemia) or >6.0 mEq/L (hyperkalemia). Both extremes cause life-threatening cardiac dysrhythmias. Place patient on cardiac monitor, check ECG, and notify the provider immediately.
What INR range is therapeutic on warfarin?
Therapeutic INR on warfarin is 2.0–3.0 for atrial fibrillation and DVT/PE. Mechanical heart valves target 2.5–3.5. INR >4.5 is critical — hold warfarin, consider vitamin K per orders.
What hemoglobin level requires a transfusion?
Transfusion is generally considered at Hgb <7 g/dL, or <8 g/dL with cardiovascular disease or active symptoms (tachycardia, dyspnea, chest pain). Critical Hgb is <7 g/dL.
What does an elevated troponin mean?
Troponin I >0.4 ng/mL is critical and indicates cardiac muscle injury (MI). Troponin rises 3–6 hours after onset and peaks at 12–24 hours. Serial troponins and ECG are standard when MI is suspected.