What is the investigation of choice for neurosyphilis?

Correct Answer: A. Venereal disease research laboratory (VDRL) test

The VDRL test is the investigation of choice for neurosyphilis because it is a non-treponemal (reagin) test that detects IgM antibodies in cerebrospinal fluid (CSF), which is essential for diagnosing CNS involvement. In neurosyphilis, the VDRL-CSF is highly specific and sensitive for detecting active infection in the CNS, particularly in general paresis of the insane (GPI), neurosyphilitic meningitis, and tabes dorsalis. The key discriminating feature is that VDRL can be performed on CSF, whereas treponemal tests (FTA-ABS, TPI) are primarily serum-based and less reliable in CSF. A positive VDRL-CSF with elevated protein and pleocytosis (lymphocytic) in CSF is pathognomonic for neurosyphilis. In Indian clinical practice, VDRL-CSF remains the gold standard for confirming CNS syphilis, especially in patients presenting with psychiatric symptoms, dementia, or spinal cord involvement. The test is cost-effective, widely available in Indian laboratories, and forms part of the standard diagnostic algorithm per Indian guidelines for syphilis management.

Why the other options are wrong

B. FTA-ABS-Fluorescent Treponemal Antibody Absorption test — FTA-ABS is a treponemal test that detects IgG antibodies and is excellent for confirming serum syphilis, but it is not suitable for CSF analysis in neurosyphilis. It remains positive lifelong even after treatment and cannot differentiate active from past infection. For neurosyphilis diagnosis, CSF VDRL (which detects IgM) is superior because it reflects current CNS inflammation. C. TPI – Treponema pallidum immobilization test — TPI is a treponemal test that is technically complex, expensive, and rarely performed in routine clinical practice, especially in India. It is not suitable for CSF testing and has been largely replaced by FTA-ABS and TP-PA. It cannot be used to diagnose neurosyphilis and is not part of standard diagnostic algorithms for CNS syphilis. D. RPR – Rapid Plasma Reagin test — RPR is a non-treponemal test similar to VDRL and is useful for screening serum syphilis, but it is not validated for CSF analysis in the same way VDRL is. VDRL is the established standard for CSF testing in neurosyphilis because of its superior sensitivity and specificity in detecting IgM antibodies in the CNS, whereas RPR is primarily a serum screening tool.

High-Yield Facts

Mnemonics

VDRL for CNS, Treponemal for Serum VDRL = Valid for Cerebrospinal fluid (non-treponemal, IgM, active infection). Treponemal tests = Treponemal (serum-based, IgG, lifelong positive). Use VDRL-CSF to diagnose neurosyphilis; use treponemal serum tests to confirm syphilis. CSF VDRL = Neurosyphilis Diagnosis When you see 'neurosyphilis' or 'CNS syphilis,' think CSF VDRL. It's the only non-treponemal test validated for CSF and detects active IgM antibodies in the brain/spinal cord. Positive VDRL-CSF + high protein + pleocytosis = neurosyphilis confirmed.

NBE Trap

NBE may pair treponemal tests (FTA-ABS, TPI) with neurosyphilis to trap students who confuse "specific/confirmatory tests" with "tests suitable for CSF diagnosis." The trap is that treponemal tests are confirmatory for serum syphilis but are not the investigation of choice for CNS involvement—only VDRL-CSF is.

Clinical Pearl

In Indian tertiary care, a patient presenting with dementia, personality change, or spinal cord signs with a history of untreated syphilis should have CSF VDRL tested immediately—a positive result with elevated protein and lymphocytic pleocytosis clinches the diagnosis of neurosyphilis (GPI or tabes dorsalis) and mandates urgent IV penicillin therapy to prevent irreversible neurological damage.

_Reference: Jawetz, Melnick & Adelberg's Medical Microbiology Ch. 27 (Treponema); Harrison's Principles of Internal Medicine Ch. 207 (Syphilis)_