During research in urolithiasis, my team has found a few apparently healthy adults with extreme (>50%) variation in creatinine excretion, compared day and night 12-hour sample (from the same day). Obviously, we suspected inadequate collection with >12 hours in the sample with higher creatinine excretion, but those volunteers insisted in a correct collection and seemed to be reliable. The volume excretion was also quite higher in the sample with more creatinine, which was the diurnal one.

Thus, we have been thinking about alternative explanations. We have thought of a possible creatinine clearance variance. Water and other fluids overload should not change significantly glomerular filtration, as this could be usually managed by ADH supression and urine dilution in a rapid way (unless >10 litres/day, which does not seem likely). Protein overload is a well-known cause of increased glomerular filtration, in clinical and experimental settings. But could this be enough for such a different creatinine excretion in a day?