Which non-treponemal syphilis test uses charcoal-labeled particles to allow macroscopic reading?

• A. RPR
• B. VDRL
• C. FTA-ABS
• D. TP-PA

Answer: (A)

The concept here is how reading method distinguishes non-treponemal tests. The rapid plasma reagin test uses charcoal-labeled particles in its antigen suspension, so when reagin antibodies in the serum react with the cardiolipin-based antigen, the resulting complexes cause visible clumping that can be read with the naked eye. That macroscopic reading is what sets this test apart from other non-treponemal tests that require a microscope to observe microflocculation. Treponemal tests, which detect antibodies to the actual T. pallidum organism, use different detection methods and do not rely on charcoal for macroscopic visualization.