Two tests for protein in urine show conflicting results: reagent strip is negative while sulfosalicylic acid (SSA) testing is 2+ positive. Which explanation best accounts for this difference?

• A. The urine contained an excessive amount of amorphous urates or phosphates that caused turbidity on SSA
• B. The urine pH was greater than 8, exceeding the buffering capacity of the reagent strip
• C. A protein other than albumin must be present in the urine
• D. The reading time of the strip was exceeded (reading at 2 minutes)

Answer: (C)

The key idea is that different protein tests in urine have different protein targets. Sulfosalicylic acid (SSA) is a broad, nonselective precipitation test: it will cloud or precipitate virtually any protein that’s in the urine. Reagent strips, on the other hand, rely on a dye-based reaction that is predominantly sensitive to albumin—the major urinary protein—in many standard strips.

So when you have a negative reagent-strip result but a strong positive SSA result, it points to the presence of nonalbumin proteins in the urine. Proteins such as Bence Jones (free light chains) or other nonalbumin proteins will precipitate with SSA and produce turbidity, but the albumin-specific pad on the urine strip may not detect them well enough to give a positive result. That mismatch is the classic clue that nonalbumin protein is driving the SSA positivity.

Turbidity from amorphous urates or phosphates could confound SSA interpretation in some cases, but it would not explain a clean negative strip paired with SSA positivity as neatly as nonalbumin protein would. Extremely high urine pH or reading the strip at an incorrect time could affect results in other ways, but they don’t best account for this specific discordance.