Comprehensive Paraneoplastic Antibody Testing in Small Cell Lung Cancer: Beyond VGCC Alone

Dr. Sen recommends against testing VGCC antibodies in isolation when CA LEMS is suspected in SCLC. SOX1 antibodies are present in approximately 60% to 70% of patients with SCLC-associated CA LEMS and, although not causative, serve as a highly specific supporting signal confirming paraneoplastic etiology. Anti-Hu (ANNA-1), the most common paraneoplastic antibody in SCLC overall, is associated with a distinct set of complications, including sensory neuropathy, encephalomyelitis, and autonomic neuropathy, and can co-occur with CA LEMS in the same patient. Testing for VGCC alone risks missing these concurrent processes, which have their own diagnostic and management implications. The practical recommendation is to order P/Q-type and N-type VGCC antibodies, SOX1, and a standard paraneoplastic reflex panel concurrently; this saves time by avoiding sequential chasing of additional tests over subsequent weeks.

Dr. Primdahl adds an important interpretive caution: false-positive paraneoplastic antibodies occur, particularly for CASPR2 and some other markers (less so for VGCC), so clinical context must anchor the interpretation of any panel result. Broad testing is preferable to narrow testing, but a positive result must always be assessed in the context of the clinical picture in front of the clinician. She also highlights that in patients on immune checkpoint inhibitors (ICIs), ICI-induced myasthenia gravis and myositis can co-occur or mimic CA LEMS, reinforcing the value of a broader panel in complex oncology cases.