Delayed Serum Creatine Kinase Elevation After Snakebite: Serial Changes and Clinical Correlates of Myotoxicity in a Korean Tertiary-Care Cohort

Background/Objectives: Initially normal serum creatine kinase (CK) activity may not exclude evolving muscle injury after a snakebite. We evaluated delayed CK elevation, clinical correlates, antivenom exposure, and hospital-day trajectories. Methods: We retrospectively analyzed 499 snakebite encounters at a tertiary hospital in South Korea. Delayed elevation was defined as serum CK ≥1000 U/L at the first or second follow-up after an initial value <1000 U/L. Incomplete follow-up was assessed using a lower bound, inverse-probability weighting (IPW), weight truncation, and outcome-model standardization. Results: Among 433 encounters with initially normal CK levels, 258 were retested, and 45 developed delayed elevation (17.4%; 95% confidence interval, 13.3–22.5%). The lower bound was 10.4%; sensitivity estimates were 14.3% with standard IPW, 17.2% with truncated IPW, and 17.3% with outcome-model standardization. Antivenom was administered in 196/499 encounters and was not independently associated with delayed elevation (adjusted odds ratio, 1.35; 95% confidence interval, 0.68–2.68). Delayed elevation was associated with local progression (40/45 vs. 27/212; p < 0.001), whereas major systemic outcomes were not more frequent (2/45 vs. 15/212; p = 0.745). Among 39 encounters measured on hospital days 1–5, the median CK level peaked on day 3 and remained above 1000 U/L on day 5. Conclusions: An initially normal CK level did not exclude later elevation among retested encounters. The population incidence remained uncertain because follow-up was clinically patterned. Delayed elevation was primarily linked to local progression rather than major systemic outcomes. Selective repeat CK testing may complement serial clinical assessments when envenomation remains active.