What problems are caused by the non-steady state of filtration markers after a change in GFR?
Accurate estimation of GFR from the serum level of an endogenous filtration marker (creatinine or cystatin C) requires a steady state; that is, the serum level is stable from day to day. This is true whether the serum level alone is used to estimate GFR or the serum level is used in an estimation equation. After a decline in GFR, the serum level rises until a new steady state is achieved. When the serum level is rising, the GFR estimate based on the non-steady state serum level overestimates the measured GFR. Conversely, after a rise in GFR, the serum level declines until a new steady state is achieved. When the serum level is declining, the GFR estimate based on the non-steady state serum level underestimates the measured GFR. In the non-steady state, the direction of change in the serum level indicates the direction of change in GFR, and the rate of change in the serum level provides some indication of the magnitude of the change in GFR.
