Serial assessment of glomerular filtration rate in lupus nephropathy

Abstract

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

Measured GFR as a Confirmatory Test for Estimated GFR

“…To block thyroidal uptake, cold iodine is administered at the time of 125 I-iothalmate administration, thus precluding its use in people with known allergies to iodine, such as shellfish or iodinated contrast media. Most 20,21,26,27,29,30 but not all [22][23][24][25] studies comparing urinary clearance of iothalamate to inulin showed a small positive bias, probably because of tubular secretion of iothalamate (Table 5).…”

Comparison of Methods for Determining Renal Function Decline in Early Autosomal Dominant Polycystic Kidney Disease

“…This can be explained by the greater discrepancy between sites with the precision of creatinine clearance than with other methods. Another study that compared longitudinal and simultaneously collected creatinine clearance with measured GFR found substantial discordance between these two methods over time (15).…”

“…Measured GFR with an exogenous marker such as inulin or iothalamate is the gold standard method, but expense and inconvenience are important drawbacks. Urine collection errors and tubular secretion of creatinine impair the precision and the accuracy of creatinine clearance as a marker of GFR (13)(14)(15). Estimating equations are inexpensive and convenient but have not been accurate in populations that are characterized by normal or near-normal renal function (16 -18).…”

Estimating Glomerular Filtration Rate

“…Previously, some studies reported that Cio overestimates Cin in normal subjects and lupus nephropathy (9,10), and 125 Iiothalamate is excreted not only by glomerular filtration but also by tubular secretion (11). Furthermore, in one of these studies (9), we calculated bias (mean/relative differences) between Cio and Cin ranging from Ϫ0.6 to 4.6/Ϫ2.7 to 23.3 and 18.8 to 26.6/17.4 to 27.1 (ml/min/1.73 m 2 /%), in two groups of subjects with mean Cin ranging from 17.6 to 24.3 and 95.9 to 108.3 ml/min/1.73 m 2 , respectively.…”