Difference between 24 hour protein excretion and PCR
What is the difference between a 24-hour protein excretion and a PCR on a random urine sample?
The 24-hour urine protein excretion represents the reference (gold standard) method. It is used universally, averages the variation in proteinuria caused by circadian rhythm, and is the most accurate for monitoring proteinuria during treatment.
However, it requires detailed instructions for urine collection and can be impractical in some circumstances (e.g., outpatient setting, infants, elderly, and incontinent patients). In practice, collection errors are common, compromising the accuracy of this “gold standard.”
The PCR on a random urine sample is the recommended alternative to the 24-hour urine collection. It is easy to obtain, is not influenced by variation in water intake, and is not subject to collection errors, and the same sample can be used for microscopic investigation.
A normal PCR is sufficient to rule out the presence of pathologic proteinuria (which decreases the number of unnecessary 24-hour urine collections). Hence a random urine PCR serves as a useful screening tool. In general, the 24-hour urine protein in grams is approximately equivalent to the urine PCR in g/g (for SI units, multiply PCR in g/mmol by ~9 to estimate g/day). However, the correlation between the random PCR and a 24-hour collection is poorer at higher levels of proteinuria (>1g/L or >0.1 g/dL), and in such cases confirmation by a 24-hour collection should be considered. The validity of a random PCR in following response to treatment in patients with glomerular disorders is also unproven. In these settings, for intraindividual comparison of trends over time (e.g., for assessment of progression or response to treatment), it is important to use the same measure, either PCR or 24-hour collection, rather than use them interchangeably.
