Calculadora de Depuração da Creatinina

Creatinine clearance estimates how much blood the kidneys filter per minute, using serum creatinine together with age, body weight and sex. It is the classic bedside estimate of kidney function and remains embedded in drug labelling: a large share of prescribing information specifies dose adjustments in terms of Cockcroft-Gault creatinine clearance rather than any newer measure, which is why the equation is still in daily use decades after better ones appeared for other purposes.

A fórmula

CrCl (mL/min) = [(140 - age) x weight] / (72 x serum creatinine)
Multiply by 0.85 if female

• age = age in years
• weight = body weight in kilograms
• serum creatinine = in mg/dL
• The 0.85 factor reflects lower average muscle mass, and therefore lower creatinine production, in women

Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron, 1976. The equation was fitted to 249 hospitalised men; the female correction factor was estimated rather than measured in that cohort.

How the calculation works

Creatinine is a waste product of muscle metabolism, produced at a roughly constant rate and cleared almost entirely by the kidneys. If production is steady, the concentration left in the blood is inversely related to how well the kidneys are filtering. The Cockcroft-Gault equation turns that relationship into a number by scaling inverse creatinine with body weight, as a proxy for muscle mass and therefore production, and subtracting age, because muscle mass and filtration both decline with age.

This is why the equation behaves oddly at the extremes of body composition. Weight enters as a direct multiplier, so using actual body weight in someone with obesity inflates the estimate, sometimes substantially. Common practice is to use ideal body weight, or adjusted body weight when actual weight exceeds ideal by more than about 30 percent, though the original paper specifies none of this and drug labels vary in what they assume.

How to read your result

The bands below are the ones used for drug dosing decisions. They are not the same as the chronic kidney disease stages, which are defined on estimated glomerular filtration rate normalised to body surface area, a different quantity computed a different way. A result here should be read as an input to a dosing question, not as a CKD stage.

• Direct oral anticoagulants, many antibiotics, metformin, gabapentin and most renally cleared chemotherapeutics have labelled thresholds stated in Cockcroft-Gault terms.
• A result close to a labelled cut-off deserves care: small differences in which weight was used can move the estimate across the boundary.

Cockcroft-Gault versus eGFR

For staging chronic kidney disease, current guidance uses estimated glomerular filtration rate from the CKD-EPI equation, which was refit in 2021 to remove the race coefficient it previously contained. eGFR is reported automatically by most laboratories alongside creatinine and is normalised to a standard body surface area of 1.73 square metres, which makes it comparable between people of different sizes but not directly usable for weight-based drug dosing.

The two numbers therefore answer different questions and will not agree. Using eGFR where a drug label specifies creatinine clearance, or the reverse, can shift a dose across a threshold. When a decision hinges on the value, and particularly at the extremes of body size or muscle mass, a measured clearance from a timed urine collection or a cystatin C based estimate is more dependable than either equation.

Limitações

• The equation assumes creatinine production is stable. It is not valid during acute kidney injury, when serum creatinine is still rising or falling and lags behind actual filtration by a day or more.
• Muscle mass drives creatinine production, so the estimate is unreliable in people who are very muscular, frail, malnourished, paraplegic, amputated, or on a vegetarian diet, all of which shift production away from what the equation assumes.
• Using actual body weight in obesity overestimates clearance. Ideal or adjusted body weight is usually substituted, but practice differs between institutions and between drug labels.
• Some drugs, including trimethoprim, cimetidine and cobicistat, block tubular secretion of creatinine. They raise serum creatinine without changing filtration, making kidney function look worse than it is.
• The original cohort was 249 hospitalised men. The female correction factor and the performance in older adults, children and pregnancy were not established in that study; the equation is not used in children at all.
• This calculator does not replace clinical assessment. Dosing decisions belong with the prescriber and pharmacist who can see the full picture, including the specific drug label.

Perguntas frequentes

What is a normal creatinine clearance?

Roughly 90 to 140 mL/min in healthy young adults, higher in men than women on average, and declining gradually with age. Values are interpreted against the person and the clinical question rather than against a fixed normal range.

Why does the formula use a different factor for women?

Women have lower average muscle mass and therefore produce less creatinine, so the same serum creatinine corresponds to better filtration. The 0.85 multiplier corrects for this. It was an estimate in the original 1976 paper rather than a value derived from female participants.

Should I use Cockcroft-Gault or eGFR?

Use eGFR for staging chronic kidney disease and Cockcroft-Gault where a drug label specifies it, which is most renally cleared medicines. They are computed differently and normalised differently, so substituting one for the other can push a dose across a labelled threshold.

Which body weight should be entered?

Practice is to use actual body weight when it is close to ideal, ideal body weight in obesity, and adjusted body weight when actual exceeds ideal by more than about 30 percent. Because weight is a direct multiplier, the choice can change the result substantially.

Can a single creatinine result tell me my kidney function?

Only approximately, and only if it is stable. A creatinine drawn while kidney function is changing rapidly lags behind reality, and creatinine alone cannot distinguish reduced filtration from reduced muscle mass. Trends across several measurements are far more informative than one value.

Referências

1. Cockcroft DW, Gault MH. Prediction of creatinine clearance from serum creatinine. Nephron, 1976.
2. Inker LA et al. New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. New England Journal of Medicine, 2021.
3. Shahbaz H, Rout P, Gupta M. Creatinine Clearance. StatPearls, NCBI Bookshelf.
4. National Kidney Foundation. GFR calculators and CKD-EPI guidance for professionals.