The tests most commonly used to estimate the status of renal function via alterations in glomerular filtration rate (GFR) are plasma creatinine concentration (PCr), blood, urea, nitrogen (BUN), and creatinine clearance (CCr). Unfortunately, these tests have some drawbacks. Nevertheless, they are quite useful to clinicians aware of their limitations, especially in many instances when it may be unnecessary or impractical to have precise measurements. Accordingly, serial measurements of PCr, especially when related to simultaneous BUN determinations, or CCr, may suffice. However, the functional status of the kidneys often needs to be corroborated by more precise methods when subtle changes in renal function are detected or when clinically indicated.