Cross-sectional comparison of standardized iothalamate clearance with four indirect methods of measuring renal function in 30 Pima Indian participants with type 2 diabetes. The 30 participants contributed a total of 144 paired measurements for which iothalamate clearance and serum for the determination of GFR by the indirect methods were collected simultaneously. The shaded regions represent error margins of ±30% for agreement between methods. The 95% distribution of differences between the methods of estimation and the reference method, expressed as percentages by the methods of Bland and Altman, were −30 to 32% for 100/cystatin C, −29 to 79% for 100/creatinine, −42 to 73% for the Cockcroft-Gault formula, and −43 to 39% for the Modification of Diet in Renal Disease (MDRD) equation.