24th February 2025

Investigation of AOSD

There is no specific diagnostic test for AOSD. Inflammatory markers such as CRP and ESR are often elevated, and there is typically leucocytosis with thrombocytosis and normocytic anaemia. Key to the diagnosis is a grossly elevated ferritin (>1000 ng/ml with a glycosylated fraction <20% giving a 90% specificity). An elevation in bilirubin, transaminases, and lactate dehydrogenase is also common. In cases of secondary HLH (approximately 40% of cases), pancytopaenia and DIC can occur (associated with a drop in ESR, low fibrinogen, high triglycerides and a markedly raised ferritin). This condition has a significant morbidity and mortality and so timely diagnosis is crucial. Gold standard is identifying haemophagocytosis in bone marrow aspirates, but features may also be evident on a blood film.

The Yamaguchi criteria were created from retrospective data and have a 93.5% sensitivity for AOSD. These include signs, symptoms and laboratory investigations. Diagnosis requires five or more criteria (two of which should be major criteria) without any of the exclusion criteria:

Table 3 Yamaguchi criteria for AOSD

X-rays in AOSD show non-specific joint space narrowing without erosions (and ankylosis over time), and so may help diagnostically but will not be as key to making the diagnosis as having a low index of suspicion clinically and requesting the appropriate laboratory investigations (particularly to exclude other conditions and screen for multi-system involvement).