Alpha 1-acid glycoprotein decreases recovery of total protein in urine when trichloroacetic acid is used to precipitate the proteins.

Total urine protein was measured in 132 samples by an automated benzethonium chloride method and the Ponceau-S/trichloroacetic acid (PS/TCA) method. Of these, 27% gave a result 0.1 g/L or more higher by the benzethonium chloride method. Of this 27%, most contained an abnormally high concentration of the acute-phase reactant, alpha 1-acid glycoprotein. By assaying urine containing added alpha 1-acid glycoprotein and albumin, we found that alpha 1-acid glycoprotein causes the PS/TCA method to underestimate the total urine protein concentration, whereas the benzethonium chloride method is unaffected. Not all urinary albumin was precipitated by TCA when alpha 1-acid glycoprotein was present. Therefore, protein methods in which trichloroacetic acid is used as a concentrating step before the assay will underestimate total urine protein when the concentration of alpha 1-acid glycoprotein is high.