A 28 year old labourer, 3 yrs back presented with penile ulcer which was not treated. Later he presented with neurological symptoms for which he got treated. Which is the test to monitor response to treatment?

This is a case of neurosyphilis. A positive nontreponemal CSF serologic test result (CSF VDRL) establishes the diagnosis of neurosyphilis (and an increased cell count in response to the spirochete documents the presence of active disease). In VDRL test, the inactivated serum is mixed with cardiolipin antigen on a special slide & rotated for 4 minutes. Cardiolipin remains as uniform crystals in normal serum but it forms visible clumps on combining with regain antibody. The reaction is read under low power microscope. By testing serial dilutions, the antibody titre can be estimated. The results are repoed as reactive, weak by reactive / not reactive. The VDRL test can be used for testing CSF also, but not plasma. A number of modification of VDRL test have been developed, of which the Rapid Plasma Reagin (RPR) is the most popular. In RPR test. VDRL antigen containing carbon paicles which make the result more clear cut & evident to the naked eye The RPR test is done with unheated serum / plasma but is not suitable for testing CSF. Ref: Ananthanarayanan & Paniker's textbook of microbiology Ed 8 Pg - 375; Klausner J.D., Hook III E.W. (2007). Chapter 20. Neurosyphilis. In J.D. Klausner, E.W. Hook III (Eds), CURRENT Diagnosis & Treatment of Sexually Transmitted Diseases.