Adult-onset Still’s disease (AOSD) can be complicated by reactive macrophage activation syndrome (rMAS). The objective of this study was to evaluate vitamin B₁₂ values in AOSD with and without rMAS.

All patients’ files with AOSD in one center were retrospectively reviewed. Hemophagocytosis was defined as phagocytosis of various hematopoietic cells by macrophages. Clinical data including fever, rash, sore throat, arthritis, lymphadenopathy were recorded. Laboratory tests included complete blood count, serum ferritin, transaminases, serum triglyceride and vitamin B₁₂ level. The control group was selected from our AOSD pool who had AOSD without rMAS.

Seven patients (5 female) had AOSD with rMAS. Median age at the diagnosis of rMAS was 32 (range, 27–37) and median follow-up duration after rMAS diagnosis was 18 months (range, 2–60). All of the patients with rMAS had fever, sore throat, rash, arthritis, anemia and hyperferritenemia. Five of seven patients had hepatosplenomegaly and lymphadenopathy. Four of seven patients had normal or low leucocyte count, three of seven patients had increased triglyceride level. The patients with AOSD and rMAS mean±standard deviation (S.D.) vitamin B₁₂ levels were significantly higher than without rMAS (1903±960 vs 542±328pg/ml, p=0.001). The specificity (75%) of increased vitamin B₁₂ level was high and sensitivity (100%) was excellent.

Elevated vitamin B₁₂ levels seems to be a good marker for diagnostic marker in AOSD when complicated with rMAS.