How to screen for homozygous complement and MBL deficiency?
In the general population, complement deficiencies are uncommon whereas up to 3% can have MBL deficiency. The total hemolytic complement assay (CH 50 ) assesses the integrity of the classic pathway of complement activation. Patient’s serum is added to a standardized suspension of sheep red blood cells (RBCs) coated with rabbit antibody. These “immune complexes” allow activation of the classic pathway, resulting in lysis of the sheep RBC. The CH 50 is the reciprocal of the serum dilution that lyses 50% of the sheep RBC. Because specific deficiencies that lead to rheumatologic manifestations are usually in the classical, rather than the alternative pathway, CH 50 is the ideal inexpensive screening test. Homozygous deficiency of a complement component in the classical pathway results in a CH 50 of 0; individual complement levels may then be determined by immunoassay. Complement component deficiencies of the classical pathway have an autosomal recessive pattern of inheritance. If the CH 50 is normal, the alternative pathway function should be tested with the AH 50 and MBL should be measured since deficiency in these components can be associated with severe infections.
