RxMath Lab Free unlimited NAPLEX and pharmacy calculation practice. Every problem is generated fresh and worked step by step.
RxMath Lab › Clinical Lab Calculation Practice

Clinical Lab Calculation Practice

Unlimited practice problems with step-by-step worked solutions. It is free and it needs no sign-up.

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  •  under 1,500 mild, under 1,000 moderate, under 500 severe neutropeniaAnion gap = Na⁺ − (Cl⁻ + HCO₃⁻)  •  normal about 8 to 12 mEq/L
Worked example

Measured 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.

Streak 0
Correct 0/0
Anion Gap
A patient has Na⁺ 135, Cl⁻ 105, and HCO₃⁻ 16 (all mEq/L). What is the anion gap? Round your answer to the nearest whole number.
mEq/L

Every problem has its own link. Hit Copy link to send a classmate this exact question, or to keep it for later.

How to set up the common lab corrections

Four small calculations turn a raw lab value into one you can act on. They aren't related to each other, but they share a feature: each is a formula that's easy to memorize and easy to use wrongly, because the trap is in when to apply it rather than in the arithmetic.

Corrected calcium

corrected Ca (mg/dL) = measured Ca + 0.8 × (4.0 − albumin in g/dL)

A large share of plasma calcium is bound to albumin, so when albumin is low the total calcium reads low even though the ionized, active fraction may be normal. The correction estimates what the total would be at a normal albumin of 4.0 g/dL. Measured calcium 7.8 mg/dL, albumin 2.2 g/dL:

corrected Ca = 7.8 + 0.8 × (4.0 − 2.2) = 7.8 + 1.44 = 9.24 → 9.2 mg/dL

Only correct when albumin is below 4.0. Above it the formula would subtract, which isn't how it's meant to be used. And if an ionized calcium is available, use that instead; the correction is a stand-in for a measurement.

Corrected phenytoin (Sheiner-Tozer)

corrected phenytoin = measured total ÷ [(0.2 × albumin) + 0.1]

Phenytoin is highly protein bound, so a total level in a hypoalbuminemic patient understates the free drug the same way total calcium does. Measured 6 mcg/mL, albumin 2.5 g/dL, normal renal function:

corrected = 6 ÷ (0.2 × 2.5 + 0.1) = 6 ÷ 0.6 = 10 mcg/mL

A level that looked subtherapeutic is in fact mid-range. In renal failure (creatinine clearance below 10 mL/min) the 0.2 becomes 0.1, because uremia displaces phenytoin from albumin as well. The question will say which situation applies.

Absolute neutrophil count

ANC = WBC × (% segs + % bands) ÷ 100

The WBC is all white cells. The neutrophils are the segmented (mature) and band (immature) forms, and only those two percentages go in. WBC 2,800 cells/mm³, 40% segs, 5% bands:

ANC = 2,800 × (40 + 5) ÷ 100 = 1,260 cells/mm³

Thresholds: under 1,500 is mild neutropenia, under 1,000 moderate, under 500 severe. A WBC reported as 2.8 × 10³/µL is the same 2,800; convert the notation before multiplying. Lymphocytes, monocytes and eosinophils are not neutrophils and don't go into the sum, however they're listed.

Anion gap

anion gap = Na⁺ − (Cl⁻ + HCO₃⁻)

Sodium 138, chloride 100, bicarbonate 14, all in mEq/L:

gap = 138 − (100 + 14) = 24 mEq/L

Normal is about 8 to 12 when potassium is left out, which is the convention here. An elevated gap means unmeasured anions are present, and the differential is the MUDPILES list (methanol, uremia, DKA, and so on). Some references include potassium on the sodium side, which raises the normal range by roughly the potassium value; use whichever form the question gives.

Where students lose the points

10 Clinical Lab Calculation practice problems with worked solutions

Every problem below is worked start to finish. Try it yourself first, then check the steps. For a fresh problem every time, use the practice box above. It generates unlimited variations of these same question types.

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
  1. Corrected Ca = measured Ca + 0.8 × (4.0 − albumin).
  2. = 8 mg/dL + 0.8 × (4.0 − 3.5 g/dL) = 8 + 0.8 × 0.5 = 8.4 mg/dL.
  3. 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
  1. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).
  2. = 140 mEq/L − (105 mEq/L + 24 mEq/L) = 140 − 129 = 11 mEq/L.
  3. 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
  1. ANC = WBC × (% segs + % bands) ÷ 100.
  2. = 2,800 cells/mm³ × (50% + 0%) ÷ 100 = 2,800 cells/mm³ × 0.5 = 1400 cells/mm³.
  3. 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
  1. Corrected = measured ÷ [(0.2 × albumin) + 0.1].
  2. = 7 mcg/mL ÷ [(0.2 × 1.8 g/dL) + 0.1] = 7 ÷ 0.46 ≈ 15.2174 mcg/mL.
  3. 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
  1. Corrected Ca = measured Ca + 0.8 × (4.0 − albumin).
  2. = 7.2 mg/dL + 0.8 × (4.0 − 3.2 g/dL) = 7.2 + 0.8 × 0.8 = 7.84 mg/dL.
  3. 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
  1. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).
  2. = 142 mEq/L − (105 mEq/L + 26 mEq/L) = 142 − 131 = 11 mEq/L.
  3. 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
  1. ANC = WBC × (% segs + % bands) ÷ 100.
  2. = 5,000 cells/mm³ × (35% + 3%) ÷ 100 = 5,000 cells/mm³ × 0.38 = 1900 cells/mm³.
  3. 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
  1. Corrected = measured ÷ [(0.2 × albumin) + 0.1].
  2. = 5 mcg/mL ÷ [(0.2 × 3.5 g/dL) + 0.1] = 5 ÷ 0.8 = 6.25 mcg/mL.
  3. 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
  1. Corrected Ca = measured Ca + 0.8 × (4.0 − albumin).
  2. = 7 mg/dL + 0.8 × (4.0 − 1.8 g/dL) = 7 + 0.8 × 2.2 = 8.76 mg/dL.
  3. 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
  1. Anion gap = Na⁺ − (Cl⁻ + HCO₃⁻).
  2. = 135 mEq/L − (108 mEq/L + 18 mEq/L) = 135 − 126 = 9 mEq/L.
  3. 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.).

Sources and how far they go

Where a convention on this page comes from a published standard, the standard is named. Where it comes from a textbook or from ordinary practice instead, the entry says so. That difference matters when a preceptor or an exam disagrees with you.

Citations are to the source of the method. They are not a review of this page, and none of these bodies has reviewed or endorsed it.

Premium is coming

Timed NAPLEX-style mock exams, saved progress, per-topic accuracy analytics, and weak-spot drills. The unlimited free practice you're using now stays free. Join the waitlist to get launch pricing when it ships.

You get one email when it launches, and nothing after that.

Practice another topic
Percentage & Ratio StrengthDilutionAlligationMilliequivalent (mEq), Millimole & MilliosmoleIV Infusion & Drip RateBody Surface Area (BSA) & Chemo DosingTPN & Nutrition CalculationIsotonicity & E-ValueCreatinine Clearance (Cockcroft-Gault)PharmacokineticsBioavailability (F)Insulin Day Supply & SwitchingOpioid Conversion & MMEDays Supply CalculationWeight-Based (mg/kg) DosingReconstitution & Powder VolumeSpecific GravityDispensing Quantity

Studying for boards is expensive enough. RxMath Lab is free, has no ads, and stays that way. If it helped you and you want to keep it running for the next class, you can support it here. That is entirely optional.