Serum creatinine is generated by muscle, and the patients in whom acute kidney injury matters most are frequently the ones with least of it. An older woman with sarcopenia, a patient with cirrhosis, someone three weeks into a critical illness - all of them can lose a large fraction of glomerular filtration while the creatinine stays inside the reference range.
The number to watch is the change from that patient's own baseline, not the position within the laboratory range. A rise from 45 to 88 micromol/L is a doubling and reports as normal twice. Urine output, where it is being measured accurately, moves earlier than creatinine in most acute injury, and a falling output with a flat creatinine is the combination most often dismissed.
So find the old value before interpreting the new one, and be explicit in the note about which baseline you are using. Where no previous result exists, say so rather than assuming the admission value is the baseline - in a patient admitted with the injury, it is not.
- Compare creatinine with the patient's own previous value, not the reference range.
- State in the note which baseline value you are using, and its date.
- Do not assume the admission creatinine is baseline in a patient admitted with the injury.
- Expect creatinine to understate loss of filtration in sarcopenia, cirrhosis and prolonged critical illness.
- Falling urine output with a flat creatinine is an early signal, not a reassuring one.
Why it matters
The patients in whom a normal creatinine is least trustworthy are exactly the ones in whom acute kidney injury does most damage.
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