Urine Protein: SSA
Protein in urine can also be estimated using sulfosalicylic acid (SSA) precipitation. The SSA reagent is added to a small volume of urine. Acidification causes precipitation of protein in the sample (seen as increasing turbidity), which is subjectively graded as trace, 1+, 2+, 3+ or 4+ (see image above).
Unlike the protein test on the dipstick, the SSA reaction will detect albumin and globulins (although it is more sensitive to albumin). In addition, the SSA detects Bence-Jones proteins (which are usually not picked up by the dipstick), although it often underestimates them.
In the past, the SSA reaction was used to confirm positive reactions for protein on the dipstick in alkaline urine. However, studies have shown that the SSA reaction (which is also a highly subjective assessment) is no more accurate (and in fact can be less accurate) than the dipstick for measurement of urine protein. For this reason, the SSA reaction is no longer routinely performed on urine samples at Cornell University. If there is concern that the urine protein on the dipstick is falsely increased in an alkaline urine sample, measurement of urine protein (or protein to urine creatinine ratio) on a chemistry analyzer is advised.
The most accurate measurement of urine protein output is measurement of urine protein excretion over 24-hours. A good alternative to this test is the urine protein to creatinine ratio.

False positives
  • Contrast media
  • Antibiotics in high concentration, e.g. penicillin and cephalosporin derivatives
  • Uncentrifuged turbid urines can look positive. Therefore, SSA should always be performed on urine supernatant.
False negatives
  • Highly buffered alkaline urine. The urine may require acidification to a pH of 7.0 before performing the SSA test.
  • Dilute urine
  • Turbid urine - may mask a positive reaction.

Cornell University