Accepted answer
An 8-point fall from a typical starting eGFR of about 95 is roughly 8 per cent, which is inside the range conventionally treated as an expected haemodynamic dip rather than as injury. The mechanism is efferent arteriolar tone and intraglomerular pressure, not nephron loss: the filtration fraction falls, the measured rate follows it down, and the trajectory afterwards is what distinguishes the two — a dip plateaus and then stabilises, injury does not. Two things confound it in the first month specifically. Fluid intake falls when food volume falls, and dehydration moves creatinine on its own. And creatinine is generated by muscle, so any change in muscle mass moves the estimate without the kidney doing anything at all. Which makes 8 points hard to interpret from one draw. A repeat under matched conditions — same hydration, same time of day, no unusual training in the preceding 48 hours — is worth more than any amount of reasoning about the first value. A cystatin C alongside it costs little and is not muscle-dependent, so a concordant pair settles the question that a creatinine alone cannot. A dip is an expectation, not a permission: it is followed rather than ignored, and nothing here is medical advice.
Start with whether you are looking at a filtration marker or a damage marker. Creatinine and cystatin C estimate filtration; albuminuria measures damage. They answer different questions.
Cystatin C is not produced by muscle, so a combined creatinine–cystatin equation is substantially more reliable in anyone whose muscle mass is atypical. It costs more and is worth it when the creatinine estimate does not fit the person.
The underlying point is that the reported eGFR is capped at ninety in many laboratories, so an apparent fall from ninety to eighty-five may be a fall from a much higher true value or from nothing at all.
Cystatin-C-based estimation is recommended as a confirmatory test where the creatinine-based estimate is likely to be misleading, which explicitly includes atypical muscle mass.
Albuminuria and filtration are different questions. Measure both.