Why isn’t a total urine volume taken over 24 hours enough to assess renal function?
A total 24-hour urine volume can be useful for identifying reduced or increased urine volume, but it is unreliable (not to mention inconvenient) and still not enough to identify renal dysfunction or failure (it can simply indicate dehydration, high fluid intake or use of diuretics) (1-3).
The measurement of urinary excretion of creatinine of the 24-hour urine volume, called creatinine clearance, reveals more because it can better indicate glomerular filtration rate (GFR) (1;2). However, creatine clearance is still limited in its precision due to possible confounding variables (1).
An approach to support proper investigation into renal function should include other measures inclindin estimated GFR (considering age, sex, ethinicity and weight) as well as tests for other markers of clearance such as cystatin C (a protein almost unaffected by meat intake), inulin (most accurate estimate of GFR) and albumin (indicator of proteinuria) (1).
Apart from GFR, in cases of possible renal tubular dysfunction measurement of tubular reabsorption is necessary because it may contribute to acute renal failure (1). Usually tubular damage occurs due to other conditions (1). Examination may include osmolality measurements of plasma and urine, water deprivation tests, urine pH and acid load tests, specific proteinuria, aminoaciduria or glycosuria tests (1).
1. Gaw A, Murphy MJ, Cowan RA, O'Reilly DStJ, Stewart MJ, Shepherd J. Clinical Biochemistry: An Illustrated Colour Text. Edinburgh: Churchill Livingstone Elsevier, 2008.
2. Adebisi SA, Adekunle BA, Etu AK. Creatinine clearance: alternative approach to traditional 24-hour urine collection in normal individuals. Afr J Med Med Sci 2001;30:27-30.
3. MedlinePlus. Urine 24-hour volume. Medical Encyclopedia. Available at: http://www.nlm.nih.gov/medlineplus/ency/article/003425.htm. Accessed on 21 June 2009.