31 Creatinine
Creatinine is a waste product formed at a fairly constant rate from creatine phosphate in muscle. It is filtered by the kidneys and excreted in urine, so its blood level rises when kidney filtration falls. Serum creatinine is the most widely used test of kidney function and is the basis for calculating eGFR and creatinine clearance. Its level also depends on muscle mass, diet and medicines, and it rises only after a substantial part of kidney function has been lost.
Creatinine and filtration are not linear
The relation between creatinine and GFR is curved (hyperbolic). When GFR halves from 120 to 60 mL/min, creatinine rises only from about 70 to about 120 µmol/L, often still inside the reference range. Later, small further losses of function cause large jumps in creatinine. That is why creatinine is converted to eGFR, which takes age and sex into account.
High creatinine
| Group | Causes |
|---|---|
| Reduced kidney function | Chronic kidney disease (diabetes, hypertension, glomerulonephritis, polycystic kidneys); acute kidney injury (dehydration, low blood pressure, sepsis, nephrotoxic drugs, contrast, urinary obstruction) |
| Higher production | Large muscle mass, intense exercise, rhabdomyolysis, cooked meat meal, creatine supplements |
| Reduced tubular secretion | Trimethoprim, cimetidine, some HIV and cancer medicines — creatinine rises without real loss of filtration |
Acute kidney injury is defined by KDIGO as a rise in creatinine of at least 26.5 µmol/L (0.3 mg/dL) within 48 hours, or to at least 1.5 times baseline within 7 days, or reduced urine output.
Low creatinine
Low values reflect low muscle mass: older age, frailty, malnutrition, muscle diseases, amputation, and also pregnancy, when filtration increases. In these situations a "normal" creatinine may hide reduced kidney function, and cystatin C can be more informative.
if creatinine rises quickly, or if it comes with markedly reduced urine output, swelling, breathlessness, confusion, nausea or vomiting.
A raised creatinine should be repeated and interpreted as eGFR together with a urine albumin test. Avoid heavy exercise and large meat meals in the 24 hours before the test, and tell your doctor about all medicines and supplements.
References
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney Int. 2024;105(4S):S117–S314. doi:10.1016/j.kint.2023.10.018
- Kidney Disease: Improving Global Outcomes (KDIGO) Acute Kidney Injury Work Group. KDIGO clinical practice guideline for acute kidney injury. Kidney Int Suppl. 2012;2(1):1–138.
- Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. N Engl J Med. 2021;385(19):1737–1749. doi:10.1056/NEJMoa2102953
This article is for general information and does not replace a consultation with a doctor. Laboratory results must be interpreted by a physician together with symptoms and other tests.
Published on 1 May 2024