Clinical Lab Calculation Practice
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Several bedside calculations adjust or interpret a lab value before you can act on it. The four here are corrected calcium for low albumin, corrected phenytoin for hypoalbuminemia (Sheiner-Tozer), the absolute neutrophil count, and the anion gap.
FormulasCorrected Ca = measured Ca + 0.8 × (4.0 − albumin)Corrected phenytoin = measured ÷ [(0.2 × albumin) + 0.1]ANC = WBC × (% segs + % bands) ÷ 100 • Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)
Worked exampleMeasured calcium 7.5 mg/dL, albumin 2.5 g/dL. Corrected = 7.5 + 0.8 × (4.0 − 2.5) = 7.5 + 1.2 = 8.7 mg/dL.
Only correct calcium when albumin is low. For phenytoin with CrCl < 10, the 0.2 factor becomes 0.1.
10 Clinical Lab Calculation practice problems with worked solutions
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Problem 1: Corrected Calcium
A patient has a measured serum calcium of 8 mg/dL and an albumin of 3.5 g/dL. What is the corrected calcium? Round your answer to one decimal place.
Answer 8.4 mg/dL
Solution- Corrected Ca = measured Ca + 0.8 × (4.0 − albumin).
- = 8 mg/dL + 0.8 × (4.0 − 3.5 g/dL) = 8 + 0.8 × 0.5 = 8.4 mg/dL.
- Rounded as the question asks: 8.4 mg/dL.
Only correct when albumin is low. Each 1 g/dL of albumin below 4.0 masks ~0.8 mg/dL of calcium.
Problem 2: Anion Gap
A patient has Na⁺ 140, Cl⁻ 105, and HCO₃⁻ 24 (all mEq/L). What is the anion gap? Round your answer to the nearest whole number.
Answer 11 mEq/L
Solution- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).
- = 140 mEq/L − (105 mEq/L + 24 mEq/L) = 140 − 129 = 11 mEq/L.
- Rounded as the question asks: 11 mEq/L.
Normal anion gap is about 8 to 12 mEq/L (potassium omitted). Elevated gap points to MUDPILES causes (lactic acidosis, DKA, salicylates, etc.).
Problem 3: ANC
A patient has a WBC of 2,800 cells/mm³ with 50% segs and 0% bands. What is the absolute neutrophil count (ANC)? Round your answer to the nearest whole number.
Answer 1,400 cells/mm³
Solution- ANC = WBC × (% segs + % bands) ÷ 100.
- = 2,800 cells/mm³ × (50% + 0%) ÷ 100 = 2,800 cells/mm³ × 0.5 = 1400 cells/mm³.
- Rounded as the question asks: 1,400 cells/mm³.
Neutrophils only, segs plus bands. Neutropenia: ANC under 1500 is mild, under 1000 moderate, under 500 severe (high infection risk).
Problem 4: Corrected Phenytoin
A patient's measured total phenytoin level is 7 mcg/mL with an albumin of 1.8 g/dL and normal renal function. What is the corrected phenytoin level? (Sheiner-Tozer) Round your answer to one decimal place.
Answer 15.2 mcg/mL
Solution- Corrected = measured ÷ [(0.2 × albumin) + 0.1].
- = 7 mcg/mL ÷ [(0.2 × 1.8 g/dL) + 0.1] = 7 ÷ 0.46 ≈ 15.2174 mcg/mL.
- Rounded as the question asks: 15.2 mcg/mL.
Sheiner-Tozer adjusts total phenytoin for hypoalbuminemia. For CrCl < 10 mL/min, the 0.2 factor becomes 0.1.
Problem 5: Corrected Calcium
A patient has a measured serum calcium of 7.2 mg/dL and an albumin of 3.2 g/dL. What is the corrected calcium? Round your answer to one decimal place.
Answer 7.8 mg/dL
Solution- Corrected Ca = measured Ca + 0.8 × (4.0 − albumin).
- = 7.2 mg/dL + 0.8 × (4.0 − 3.2 g/dL) = 7.2 + 0.8 × 0.8 = 7.84 mg/dL.
- Rounded as the question asks: 7.8 mg/dL.
Only correct when albumin is low. Each 1 g/dL of albumin below 4.0 masks ~0.8 mg/dL of calcium.
Problem 6: Anion Gap
A patient has Na⁺ 142, Cl⁻ 105, and HCO₃⁻ 26 (all mEq/L). What is the anion gap? Round your answer to the nearest whole number.
Answer 11 mEq/L
Solution- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).
- = 142 mEq/L − (105 mEq/L + 26 mEq/L) = 142 − 131 = 11 mEq/L.
- Rounded as the question asks: 11 mEq/L.
Normal anion gap is about 8 to 12 mEq/L (potassium omitted). Elevated gap points to MUDPILES causes (lactic acidosis, DKA, salicylates, etc.).
Problem 7: ANC
A patient has a WBC of 5,000 cells/mm³ with 35% segs and 3% bands. What is the absolute neutrophil count (ANC)? Round your answer to the nearest whole number.
Answer 1,900 cells/mm³
Solution- ANC = WBC × (% segs + % bands) ÷ 100.
- = 5,000 cells/mm³ × (35% + 3%) ÷ 100 = 5,000 cells/mm³ × 0.38 = 1900 cells/mm³.
- Rounded as the question asks: 1,900 cells/mm³.
Neutrophils only, segs plus bands. Neutropenia: ANC under 1500 is mild, under 1000 moderate, under 500 severe (high infection risk).
Problem 8: Corrected Phenytoin
A patient's measured total phenytoin level is 5 mcg/mL with an albumin of 3.5 g/dL and normal renal function. What is the corrected phenytoin level? (Sheiner-Tozer) Round your answer to one decimal place.
Answer 6.3 mcg/mL
Solution- Corrected = measured ÷ [(0.2 × albumin) + 0.1].
- = 5 mcg/mL ÷ [(0.2 × 3.5 g/dL) + 0.1] = 5 ÷ 0.8 = 6.25 mcg/mL.
- Rounded as the question asks: 6.3 mcg/mL.
Sheiner-Tozer adjusts total phenytoin for hypoalbuminemia. For CrCl < 10 mL/min, the 0.2 factor becomes 0.1.
Problem 9: Corrected Calcium
A patient has a measured serum calcium of 7 mg/dL and an albumin of 1.8 g/dL. What is the corrected calcium? Round your answer to one decimal place.
Answer 8.8 mg/dL
Solution- Corrected Ca = measured Ca + 0.8 × (4.0 − albumin).
- = 7 mg/dL + 0.8 × (4.0 − 1.8 g/dL) = 7 + 0.8 × 2.2 = 8.76 mg/dL.
- Rounded as the question asks: 8.8 mg/dL.
Only correct when albumin is low. Each 1 g/dL of albumin below 4.0 masks ~0.8 mg/dL of calcium.
Problem 10: Anion Gap
A patient has Na⁺ 135, Cl⁻ 108, and HCO₃⁻ 18 (all mEq/L). What is the anion gap? Round your answer to the nearest whole number.
Answer 9 mEq/L
Solution- Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).
- = 135 mEq/L − (108 mEq/L + 18 mEq/L) = 135 − 126 = 9 mEq/L.
- Rounded as the question asks: 9 mEq/L.
Normal anion gap is about 8 to 12 mEq/L (potassium omitted). Elevated gap points to MUDPILES causes (lactic acidosis, DKA, salicylates, etc.).