CH50

CH50, or total hemolytic complement activity, is a functional assay that assesses the capacity of a person’s serum to lyse antibody-coated sheep red blood cells via the classical complement pathway. It reflects the integrity of the classical pathway components, including C1q, C1r, C1s, C4, and C2, and the downstream terminal components. The result is expressed as the serum dilution that yields 50% lysis under standardized conditions, reported in CH50 units per milliliter (U/mL). Normal values are laboratory-dependent but commonly fall around 40–60 U/mL.

The assay procedure uses a fixed amount of antibody-sensitized sheep erythrocytes incubated with serial dilutions of patient serum and a standardized source of complement, typically guinea pig serum. After incubation, lysis is measured spectrophotometrically. The CH50 is defined as the reciprocal of the dilution that produces 50% lysis. In principle, higher functional complement activity yields a higher CH50 value, whereas deficiencies or consumption of classical pathway components lower the CH50.

Interpretation of CH50 results is context-dependent. A reduced CH50 points to deficiency or consumption of the classical pathway components and can result from hereditary deficiencies (for example, in C1q, C4, or C2) or from immune complex–mediated disease. CH50 is commonly interpreted with measurements of C3 and C4 and with the AH50, an assay of the alternative pathway. Normal CH50 with low C3 may suggest predominant involvement of the alternative pathway or isolated component issues; a low CH50 with low C4 and C3 supports classical pathway consumption.

CH50 is used in the evaluation of suspected complement deficiencies and in monitoring conditions or therapies that affect the classical pathway. Results are subject to pre-analytical and methodological variability and should be interpreted within the broader clinical and laboratory context.