Sodium, potassium, urea, creatinine and bicarbonate, read together
Expansion
Read in this order
- Creatinine and the trend, since the baseline determines whether this is acute
- Urea, and the urea to creatinine ratio
- Sodium, interpreted with the volume status
- Potassium, checking for haemolysis if unexpectedly high
- Bicarbonate, which is a clue to an unmeasured acid-base disorder
Urea to creatinine ratio
| Pattern | Suggests |
|---|---|
| Urea disproportionately high | Hypovolaemia, upper gastrointestinal bleed, high protein intake, steroids, catabolism |
| Both raised proportionately | Intrinsic renal disease |
| Urea disproportionately low | Liver failure, low protein intake, pregnancy, overhydration |
Creatinine and eGFR pitfalls: creatinine depends on muscle mass, so it underestimates dysfunction in the elderly, in amputees and in cachexia, and overestimates it in body builders and after a large meat meal. It lags behind a falling glomerular filtration rate by 24 to 48 hours, so an early acute kidney injury has a normal creatinine. Trimethoprim and cimetidine raise it by blocking tubular secretion without changing function.
eGFR assumes a steady state and is invalid in acute kidney injury, in pregnancy, and at extremes of body size, and it is not validated for drug dosing in these groups.
A low bicarbonate points to a metabolic acidosis and should prompt calculation of the anion gap; a high bicarbonate suggests a metabolic alkalosis or chronic carbon dioxide retention.
Spurious hyperkalaemia from haemolysis, a tight tourniquet, fist clenching, a delayed sample or a very high platelet or white cell count is common and must be excluded before treating.