CalRegional Phlebotomy Practice Test

Creatinine clearance uses a 24-hour urine collection to estimate how well the kidneys are filtering blood. Creatinine, a waste product produced at a relatively steady rate by muscle, is filtered by the kidneys and excreted in urine with little reabsorption. By measuring how much creatinine is excreted in urine over an entire day and comparing it with the creatinine level in the blood, you get an estimate of the glomerular filtration rate (GFR), which reflects overall kidney function. That 24-hour collection is key because it provides the total amount cleared in a day, allowing calculation of clearance in milliliters per minute. For this test to be accurate, the collection must be truly complete over the full 24 hours; missing urine or delays can skew results. While newer practice often uses estimates like eGFR from a single blood sample, creatinine clearance from a 24-hour urine specimen remains a direct, practical way to assess kidney filtration capacity. The other options measure different physiological processes—glucose tolerance tests carbohydrate metabolism after a glucose load, fibrin split products relate to coagulation and fibrinolysis, and culture and sensitivity identify pathogens and guide antibiotics—so they are not about kidney filtration in a 24-hour urine context.