High soluble interleukin-2 receptor values in Indian paediatric & adult controls - Need for population-specific threshold in the diagnosis of haemophagocytic lymphohistiocytosis.

Background & objectives: Elevated soluble interleukin-2 receptor (sIL2R) is a diagnostic criterion for haemophagocytic lymphohistiocytosis (HLH). International guidelines propose a 2400 U/ml cut-off or individual laboratory-defined cut-off. However, sIL2R normal values are so far not known in Indians. So, this study was undertaken to measure sIL2R in healthy children and adults to establish age-related reference values. Methods: Healthy controls and cases (participants with persistent fever, organomegaly, cytopenias and biochemical markers of HLH) were prospectively enrolled. Serum sIL2R was measured by double-sandwich enzyme immunoassay in a standardization batch to determine the optimum cut-off value using receiver operator characteristic curve and was subsequently validated. Results: One hundred and forty six age- and sex-matched children (80 controls and 66 suspected HLH cases) and 55 adults (49 controls and 6 suspected HLH cases) were prospectively enrolled. The optimal sIL2R cut-off ≥23 ng/ml was defined as raised sIL2R in the standardization batch. No controls had sIL2R ≥23 ng/ml in the validation batch. In healthy controls, median sIL2R (interquartile range) decreased with increasing age from 9.0 ng/ml (6.6-13.4) below five years of age to 3.2 ng/ml (2.8-5.1) in adults. Proposed upper limit of normal value for sIL2R is 17.4 ng/ml in less than five year, 12.2 ng/ml in 5-9 yr, 6.7 ng/ml in 10-17 yr and 5.2 ng/ml in ≥18 yr. sIL2R accuracy to diagnose HLH marginally improved with age-appropriate cut-off. Interpretation & conclusions: Paediatric controls in India showed higher sIL2R levels than most studies conducted in other countries, except for some reports in Chinese and Russian populations. Age-appropriate reference values of sIL2R in a specific population may be considered to determine elevated sIL2R as a marker of HLH.