Module 11 of 14 · 14 min
Renal function & body size
Most renal dose adjustments (antibiotics, anticoagulants, metformin) start from a Cockcroft–Gault creatinine clearance, and exams test whether you pick the right weight and the right constants for the units given.
🧠 "1.23 for him, 1.04 for her" in SI; "72 and 0.85" in mg/dL — never mix the pairs.
📌 Memorize these
- SCr
- $1\,\text{mg/dL} = 88.4\,\mu\text{mol/L}$
- C–G conventional
- $\div 72\times\text{SCr(mg/dL)}$female × 0.85
- C–G SI
- $\times 1.23$ (male) / $\times 1.04$ (female)SCr in µmol/L
- IBW (Devine)
- 50 kg ♂ / 45.5 kg ♀ + 2.3 kg per inch over 5 ft
- AdjBW
- $\text{IBW} + 0.4(\text{TBW}-\text{IBW})$
- $1\,\text{in}$
- $2.54\,\text{cm}$
Cockcroft–Gault comes in two forms that must not be mixed. Conventional: SCr in mg/dL, constant 72, female × 0.85. SI: SCr in µmol/L, multiply by 1.23 for males or 1.04 for females. $1\ \text{mg/dL}=88.4\ \mu\text{mol/L}$.
Ideal body weight (Devine): $\text{IBW}=50\ \text{kg (male) or }45.5\ \text{kg (female)}+2.3\ \text{kg per inch over 5 ft}$.
Which weight? A common rule (the one used here unless a question says otherwise): use actual weight if it is below IBW; use IBW if actual is up to 120% of IBW; if actual is more than 120% of IBW use adjusted weight $\text{AdjBW}=\text{IBW}+0.4(\text{TBW}-\text{IBW})$. References vary, so follow the rule the question gives.
BMI $=\dfrac{\text{kg}}{\text{m}^2}$; rearranged, the weight for a target BMI is $\text{BMI}\times\text{height}^2$.
🧭 The method
- 1Convert height to inches (or metres) and SCr to the units of the formula you will use.
- 2Calculate IBW; compare total body weight with IBW (ratio).
- 3Choose the dosing weight (TBW, IBW or AdjBW) per the stated rule.
- 4Plug into the matching Cockcroft–Gault form; apply the female factor (0.85 or 1.04) if female.
- 5Report mL/min and sense-check (young healthy adult ≈ 100–120 mL/min).
Before each reveal, predict the next step — then check.
A 68-year-old man, 5 ft 10 in, 92 kg, SCr 110 µmol/L (1.24 mg/dL). Estimate CrCl with Cockcroft–Gault (SI form), using AdjBW if TBW > 120% of IBW.
- 1
IBW: 10 inches over 5 ft.
$$\text{IBW}=50+2.3\times10=73\ \text{kg}$$ Compare TBW with IBW: more than 120%, so use AdjBW.
$$\frac{92}{73}=1.26\ (126\%)$$Adjusted body weight.
$$\text{AdjBW}=73+0.4(92-73)=80.6\ \text{kg}$$SI Cockcroft–Gault (male × 1.23, SCr in µmol/L).
$$\text{CrCl}=\frac{(140-68)\times80.6\times1.23}{110}=\frac{7137.9}{110}=64.9\ \text{mL/min}$$
Same method — the last steps are yours. Work on paper, then reveal.
A 75-year-old woman weighing 60 kg (equal to her IBW), SCr 1.2 mg/dL (106 µmol/L). Estimate CrCl with the conventional formula.
- 1
Numerator.
$$(140-75)\times60=3900$$ - 2
Denominator (conventional constant 72, SCr in mg/dL).
$$72\times1.2=86.4$$ - 3
Your turn
Now you: divide, then apply the female factor.
Work it out on paper, then reveal.
$$\frac{3900}{86.4}\times0.85=45.14\times0.85=38.4\ \text{mL/min}$$
Type your number. Your first try decides when this module comes back for review.
Convert a serum creatinine of 1.5 mg/dL to µmol/L (1 mg/dL = 88.4 µmol/L).
Multiply by 88.4.
= 132.6 µmol/L
What is the BMI of a patient who weighs 82 kg and is 1.75 m tall?
Divide weight by height squared.
= 26.8 kg/m²
A patient with anorexia is 5 ft 4 in tall. What is the minimum weight (kg) for a BMI of 18.5? (1 in = 2.54 cm)
Height in metres first: 64 in × 0.0254. Then weight = BMI × height².
= 48.9 kg
What is the ideal body weight of a woman who is 5 ft 6 in tall?
Female base is 45.5 kg; add 2.3 kg per inch over 60 in.
= 59.3 kg
An 80-year-old woman weighing 52 kg (use actual weight) has SCr 95 µmol/L. Estimate CrCl with the SI Cockcroft–Gault formula.
SI form for a woman: multiply by 1.04 (not 1.23, and do not also apply 0.85).
= 34.2 mL/min
A 45-year-old man, 6 ft 0 in, 130 kg, SCr 0.9 mg/dL. Using AdjBW (because TBW > 120% IBW), estimate CrCl with the conventional formula.
IBW = 50 + 2.3 × 12. Using total body weight here overestimates CrCl (≈ 191 mL/min).
= 144.5 mL/min
⚠️ Tricky
A 68-year-old man weighs 154 lb (1 kg = 2.2 lb), is 5 ft 10 in tall (IBW 73 kg) and has SCr 1.2 mg/dL. His actual weight is below his IBW, so use actual weight. Estimate his CrCl with the conventional Cockcroft–Gault formula.
Convert pounds to kg first; the IBW is given only so you can see that actual weight is lower.
= 58.3 mL/min
⚠️ The trap: Putting 154 (pounds) into the formula gives 128.3 mL/min, more than double the real clearance.
⚠️ Tricky
Weight rule: use IBW if total body weight is between IBW and 120% of IBW; use AdjBW if above 120%. A 55-year-old man is 5 ft 9 in tall, weighs 78 kg and has SCr 1.0 mg/dL. Estimate CrCl (conventional formula).
IBW = 50 + 2.3 × inches over 60. Compare 78 kg with 120% of IBW before choosing.
= 83.5 mL/min
⚠️ The trap: Using total body weight (78 kg) when the rule calls for IBW gives 92.1 mL/min.
⚠️ Tricky
Hospital policy: for patients aged 65 or over with SCr below 1.0 mg/dL, round SCr up to 1.0 mg/dL in Cockcroft–Gault. An 82-year-old woman, 5 ft 5 in tall (IBW 57 kg), weighs 50 kg and has SCr 0.6 mg/dL. Using actual weight (it is below IBW), estimate CrCl.
Apply the policy before calculating; women get × 0.85 in the conventional formula.
= 34.2 mL/min
⚠️ The trap: Using the measured 0.6 mg/dL gives 57.1 mL/min, which overstates the clearance of a frail elderly patient with little muscle mass.
⚠️ Tricky
Dosing table for a renally cleared antibiotic: CrCl above 50 mL/min → 1 g q8h; 25–50 → 1 g q12h; 10 to under 25 → 500 mg q12h. An 80-year-old man weighing 60 kg has SCr 158 µmol/L. Using the SI Cockcroft–Gault formula (× 1.23 for men), what total daily dose (mg/day) should he receive?
SCr is in µmol/L, so keep the 1.23 constant; then read the band and convert the regimen to mg per day.
= 2000 mg/day
⚠️ The trap: Dropping the 1.23 constant gives 22.8 mL/min, the wrong band, and only 1000 mg/day.
⚠️ Tricky
The lab reports an eGFR of 45 mL/min/1.73 m² for a man who is 185 cm tall and weighs 115 kg. A drug label gives its renal cut-offs as absolute GFR in mL/min. Using Mosteller BSA $=\sqrt{\text{cm}\times\text{kg}/3600}$, what is his absolute GFR?
eGFR is normalized to 1.73 m²; to de-normalize, multiply by the patient's BSA ÷ 1.73.
= 63.2 mL/min
⚠️ The trap: Reading the normalized 45 mL/min/1.73 m² as if it were his absolute GFR underestimates a large patient's clearance (63.2 mL/min) and can push him into a lower dosing band.
🕵️ Spot the mistake
A 60-year-old man, 70 kg (equal to his IBW), SCr 88 µmol/L. Estimate CrCl.
A student wrote
⚠️ Traps examiners set
- Using 72 with SCr in µmol/L (or 1.23 with mg/dL) — match constants to units.
- Forgetting the female factor, or applying 0.85 in the SI form that already uses 1.04.
- Using total body weight in an obese patient, which overestimates CrCl.
- Forgetting to convert feet/inches to inches over 60 (IBW) or to metres (BMI).
- Using height in cm instead of m when calculating BMI.
💬 Explain it in your own words
Why does using total body weight overestimate creatinine clearance in an obese patient, and why is adjusted weight a compromise rather than IBW alone?