Your measured creatinine clearance provides a direct assessment of kidney filtration capacity, expressed in mL/min. Normal values are approximately 90-140 mL/min for adult males and 80-125 mL/min for adult females, though these decline with age. A result below 60 mL/min generally indicates moderate kidney impairment and may correspond to CKD stage 3 or higher. Values below 15 mL/min suggest severe kidney failure that may require renal replacement therapy.

Measured creatinine clearance slightly overestimates the true glomerular filtration rate (GFR) because a small amount of creatinine is secreted by the renal tubules in addition to being filtered at the glomerulus. This overestimation is typically around 10-15% in patients with normal kidney function but becomes more pronounced as kidney function declines, because the proportion of secreted creatinine increases relative to filtered creatinine.

To validate the accuracy of your result, check whether the 24-hour urine creatinine excretion falls within the expected range: 20-25 mg/kg/day for males and 15-20 mg/kg/day for females. If the measured excretion is significantly below these values, the urine collection was likely incomplete, and the calculated clearance will be falsely low.

Use the 24-hour creatinine clearance when estimated GFR (eGFR) equations such as CKD-EPI or MDRD may be inaccurate for a specific patient. Key situations include extremes of body size (BMI above 40 or very low BMI), unusual muscle mass (bodybuilders, amputees, paraplegic patients), patients on vegetarian or vegan diets (which lower serum creatinine independent of kidney function), and during pregnancy when physiologic changes alter creatinine metabolism.

Measured CrCl is particularly important for precise drug dosing in nephrotoxic or narrow-therapeutic-index medications. Carboplatin dosing, for example, is directly calculated from measured creatinine clearance using the Calvert formula. Other scenarios include evaluation of potential kidney donors, monitoring kidney function in patients receiving chronic nephrotoxic medications (e.g., calcineurin inhibitors, aminoglycosides), and in cases where trending kidney function over time requires more precision than eGFR provides.

The primary limitation is the accuracy of the urine collection itself. Incomplete collections are extremely common — studies suggest that up to 30% of 24-hour urine collections are inadequate. Patient non-compliance (missing a void, discarding urine) leads to falsely low creatinine clearance. Overcollection (including urine from before the start time) leads to falsely high values. Always check urine creatinine excretion per kg to assess collection adequacy.

As mentioned, measured CrCl overestimates true GFR due to tubular creatinine secretion. This overestimation increases in advanced CKD. In some clinical contexts, cimetidine is administered to block tubular secretion and improve accuracy, though this is rarely done outside of research settings.

The test is also inconvenient for patients and logistically challenging. It requires a full 24-hour collection period with careful instructions, which many patients find burdensome. Laboratory processing errors, sample mislabeling, and inaccurate volume measurement can all affect results. For most routine clinical purposes, eGFR equations provide adequate accuracy, and the 24-hour collection should be reserved for situations where the added precision justifies the effort.