Urine albumin and protein measurement

Continuing the theme of must knows regarding urinalysis:

Urine albumin is detected by the urine dipstick via a reaction with tetrabromophenol blue. The reaction produces a colorimetric result depending on the amount of albumin present, which can be semi-quantitated as 1+, 2+ or 3+ by the user.

Albumin first becomes detected by the urine dipstick method when daily excretions are greater than 300mg per day. Therefore, it is insensitive to microalbuminuria (which itself has been independently associated with adverse cardiovascular outcomes).

However, it is important to note that the detection is inherently related to the concentration of albumin in the sample. Therefore, a concentrated urine sample will have a higher reading than a dilute sample, even though the total albumin loss per day may be the same. This speaks to the preference to collect samples in the same way, same time of day etc. for reproducibility purposes. Generally a first morning sample has been shown to have the closest correlation with 24hr urine collections.

In cases of plasma cell dyscrasia that produce large amounts of non-albumin protein, the urine disptick may not give an accurate reflection of the true degree of proteinuria. An older method to detect non-albumin proteins was to add sulphosalicylic acid to the dipstick negative sample and watch for a change in turbidity, indicating the presence of protein. Another useful trick is to compare the urine microalbumin/creatinine ratio and the urine protein/creatinine ratio, looking for a large gap between the two.