What is the gold standard for quantification of proteinuria?

Updated: Mar 25, 2020
  • Author: Beje Thomas, MD; Chief Editor: Vecihi Batuman, MD, FASN  more...
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Answer

The gold standard for quantification of proteinuria is the 24-hour urine collection. The normal amount of protein in the urine is <150 mg/day.

The 24-hour urine collection is performed by voiding upon waking and then collecting all urine on subsequent voids until the first void of the next day.  Obviously, the process can be cumbersome and inaccurate. Results are considered reliable based on comparison with the typical amount of creatinine secreted per kilogram of lean body mass. On average, males secrete 20-25 mg/kg per day and females secrete 15-20 mg/kg. However, after the age of 50 years, lean body muscle mass is lost, so these estimates can be inaccurate in older patients. Another option—possibly more accurate, as it accounts for race and sex—is the following calculation (can be calculated with or without phosphorus):

Estimated creatinne excretion (mg/day) = 1115.89 + (11.97 x weight in kg) - (5.83 x age) - (60.18 x phosphorus in mg/dL) + (52.82 if black) - (368.75 if female) 

The spot albumin or protein–to-creatinine ratio was developed to help make the quantification of proteinuria easier and less laborious. However, the ratio can vary depending on the time of day and the amount of creatinine excreted. Consequently, the patient should collect all samples at about the same time of day. The amount of creatinine excretion to adequately reflect a 24-hour urine collection should be about 1 gram. If it significantly less, that could lead to underestimation of the degree of proteinuria, while overestimation may occur if there is much more than 1 g of creatinine.

A spot protein or albumin–to-creatinine ratio of >3-3.5 mg protein/mg creatnine or a 24-hour urine collection showing >3-3.5 g of protein is nephrotic-range proteinuria.

Screening for proteinuria can be done using a urine dipstick or early morning spot protein or albumin–to-creatinine ratio. If significant proteinuria is found or the clinical situation is suspicious for significant proteinuria, a 24-hour urine collection should be done. The spot albumin or protein–to-creatinine ratio can be used for followup. If the ratio shows a significant increase, the 24-hour urine collection should be repeated. [5, 26, 27]


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