Imprecision of urinary iothalamate clearance as a gold-standard measure of GFR decreases the diagnostic accuracy of kidney function estimating equations.

BACKGROUND Evaluating the accuracy of estimated glomerular filtration rate (eGFR) derived from serum creatinine (SCr) and serum cystatin C (SCysC) equations requires gold-standard measures of GFR. However, the influence of imprecise measured GFRs (mGFRs) on estimates of equation error is unknown. STUDY DESIGN Diagnostic test study. SETTING & PARTICIPANTS 1,995 participants from the Modification of Diet in Renal Disease (MDRD) Study and African American Study of Kidney Disease and Hypertension (AASK) with at least 2 baseline mGFRs from iodine 125-iothalamate urinary clearances, 1 standardized SCr value, and 1 SCysC value. INDEX TESTS eGFRs calculated using the 4-variable isotope-dilution mass spectrometry (IDMS)-traceable MDRD Study equation, the Chronic Kidney Disease (CKD) Epidemiology Collaboration (CKD-EPI) SCysC equation, the CKD-EPI SCr-SCysC equation, and mGFRs collected from another prerandomization visit. REFERENCE TESTS A single reference mGFR, average of 2, and average of 3 mGFRs; additional analysis limited to consistent mGFRs (difference 30%. Reducing and quantifying errors in gold-standard measurements of GFR is critical to fully estimating the accuracy of GFR estimates.