In patients with lupus nephropathy (LN), previous studies have shown that creatinine clearance (C(Cr)) overestimates true glomerular filtration rate as measured by inulin clearance (C(In)), and that among patients the degree of overestimation is highly variable. We sought to determine whether the discrepancy between C(Cr)) and C(In) remains constant over time (months, years) in each individual patient, and therefore whether serial measurements of C(Cr) reliably reflect the direction and magnitude of change in C(In). Twenty-five patients with LN underwent simultaneous determinations of C(Cr) and C(In) performed two to four (mean 3.3) times over three years. In a given patient, it was found that the ratio of C(Cr)/C(In) changed substantially over time (mean SD 0.16 with 95% confidence interval of 0.12 to 0.20). Thus, in about 32% of cases the ratio of C(Cr)/CIn) will vary more than ± 16% from a previously measured value of C(Cr)/C(In). Patients with both high and low values of C(In) showed similar variability in C(Cr)/C(In) over time. Variability in C(Cr)/C(In) was found regardless of whether C(In) was increasing, decreasing, or constant over time. In nearly one-half of all measurements of C(Cr), the corresponding change in C(In) was directionally discordant. Iothalamate and technetium-DTPA renal clearances correlated highly with C(In) (R2 = 0.99). We conclude that the discrepancy between C(Cr) and C(In) can vary greatly over time in an individual patient. Consequently, serial C(Cr) does not accurately measure the direction or magnitude of change in glomerular filtration rate in lupus nephropathy.
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