Clinical Chemistry: 24 Hour Urine Collections and Formulas

Urine containers should be picked up from Central Supply when 24-hour urine collections are required, as designated under the "Specimen Requirements" column in the list of tests" Container 1, used for routine chemistry tests, is a chemically clean 1 gallon polyethylene bottle.

Special Urine Collections

Creatinine Clearance:
Creatinine clearance reflects glomerular filtration rate. The equation for the calculation of creatinine clearance is as follows:

Urine Creat (mg/dl)             Vol of Urine {ml)
---------------------X------------------------ 
Serum Creat (mg/dl)        Collection Period (min)

A timed specimen is necessary, but not necessarily a 24 hr collection {minimum of 6 hours collection).

Fractional Excretion of Filtered Sodium:
This is a sensitive and specific test for acute tubular necrosis, (Espinel, C.H. and Gregory, A,W., Clin. Nephrol. 13,73-77,1980). The measurements that are done are as follows: Serum and urine sodium, serum and urine creatinine; no timed specimen is necessary. The excretion fraction of filtered sodium is given by the equation.

              Urine Na          Plasma Cr
FeNa = -------------X-------------- X 100%
              Plasma Na        Urine Cr

Amylase Clearance / Creatinine Clearance:
Amylase clearance is increased in patients with acute pancreatitis but is normal in other causes of hyperamylasemia. Amylase clearance is useful in differentiating the causes of increased serum amylase; it is especially useful in confirming or excluding the diagnosis of acute pancreatitis. In macroamylasemia, the clearance is lower, usually less than 1olo. Amylase clearance is the renal clearance of amylase expressed as a percentage of creatinine clearance, and is given by simplified equation:

                 Urine Amylase                   Plasma Creatinine
ACCR = -----------------------X------------------------------   
                 Urine Creatinine                Plasma Amylase

As seen by inspection of the above equation, the clearance ratio, expressed as percentage, is calculated simply from the concentration of amylase and creatinine in serum and urine samples obtained simultaneously. No timed collections are necessary.