Creatinine Clearance (Cockcroft-Gault)

The Cockcroft-Gault equation estimates creatinine clearance, a measure of kidney function, from routine bedside values. It remains the reference method for adjusting drug doses in renal impairment because so many drug labels were written against it.

How it works

The equation, in conventional US units, is:

CrCl (mL/min) = ((140 - age) * weight_kg) / (72 * SCr_mg_dL)
             *  0.85 if the patient is female

If serum creatinine is reported in µmol/L (SI units), it is first converted to mg/dL by dividing by 88.42. The tool then maps the result to a function band so you can see at a glance whether clearance is normal, mildly, moderately, or severely reduced.

Worked example

A 65-year-old man weighing 80 kg with a serum creatinine of 1.1 mg/dL:

CrCl = ((140 − 65) × 80) ÷ (72 × 1.1)
     = (75 × 80) ÷ 79.2
     = 6000 ÷ 79.2
     ≈ 75.8 mL/min

This falls in the mild-reduction band (roughly 60–89 mL/min), which in many drug monographs means standard dosing or modest adjustment.

For a female patient with the same parameters, multiply by 0.85: 75.8 × 0.85 ≈ 64.4 mL/min — still mild reduction, but closer to the 60 mL/min threshold that triggers adjustments for some renally-cleared drugs.

Interpreting the result: CrCl bands

Creatinine clearance results are grouped into bands that align with common dose-adjustment thresholds. Exact definitions vary by guideline, but a commonly used framework is:

Many drug labels use their own thresholds (e.g. above 50, 30–50, 10–30, below 10) that differ from CKD staging. Always use the threshold specified in the drug’s prescribing information rather than a generic band.

Cockcroft-Gault vs eGFR: which to use for dosing

Pharmacists and prescribers often ask whether to use Cockcroft-Gault CrCl or the more modern CKD-EPI or MDRD equations (which produce eGFR). The answer depends on context:

For drug dosing: Use Cockcroft-Gault CrCl when the prescribing information specifies it — and most older and many newer drug labels do, because the dose-finding studies used Cockcroft-Gault. Using eGFR to apply a CrCl-based threshold introduces a systematic inconsistency.

For CKD staging and disease monitoring: The CKD-EPI equation is preferred by nephrologists and guidelines because it is more accurate across a broader population range. Most labs now report eGFR using CKD-EPI.

The practical issue: For a given patient, Cockcroft-Gault CrCl and CKD-EPI eGFR are often similar but not identical — differences of 10–20 mL/min are common, and they are conceptually different measures (clearance vs filtration rate). For drugs with critical renal thresholds, this difference can be clinically significant.

Body weight considerations

The original Cockcroft-Gault equation uses actual body weight. For patients who are obese or underweight, this can lead to over- or under-estimation of renal function:

• Obese patients: Using actual body weight overestimates CrCl because excess adipose tissue does not generate proportionally more creatinine. Many references recommend using ideal body weight (IBW) or adjusted body weight (AjBW) for patients who are significantly overweight.
• Underweight or cachectic patients: Actual weight may underestimate lean muscle mass in some scenarios, though this is less commonly corrected.

The tool uses whatever weight you enter, so apply your institution’s policy before entering the value.

Notes

Treat the result as an estimate to confirm clinically, not a diagnosis. Serum creatinine can be transiently elevated by dehydration, rhabdomyolysis, or certain medications; it can be artificially low in the very elderly or malnourished due to low muscle mass. In these cases, the Cockcroft-Gault result may not reflect true renal clearance even if it falls in a normal band.