Complement Testing

The clinical presentation of thrombotic microangiopathy (TMA) has been associated with multiple genetic disease including atypical hemolytic uremic syndrome (aHUS), thrombotic thrombocytopenic purpura (TTP), C3 glomerulopathy (C3G, comprising dense deposit disease and C3 glomerulonephritis), congenital B12 deficiency and others. These are difficult-to-diagnose, very sick patients with distinct treatment depending on the nature of the TMA. Accurate, rapid diagnosis is critical. Additionally, patients with the C5 p.Arg885 polymorphism may respond poorly to the current approved therapy, eculizumab. Note: this is the third time (3.0 name designation) we have updated the gene list to reflect the most current understanding of aHUS/TMA in the scientific literature.

Factor H is a regulator of the alternative complement activation pathway. Acquired Factor H deficiency due to autoantibody production can lead to overactive complement. Continuous complement activation is associated with development of atypical Hemolytic Uremic Syndrome (aHUS) and Dense Deposit Disease (DDD).

C3 Nephritic Factor is an autoantibody. It stabilizes C3 convertase. C3 Nephritic Factor is associated with approximately 80 percent of patients with DDD; it is also seen in approximately 40 percent of C3GN patients. It's not specific; however, detection of C3 Nephritic Factor supports the diagnosis of C3GN or DDD.

The CH50 is the most common assay to measure total complement activity in the classical complement pathway (C1 through C9) by adding patient serum to sheep RBC coated with anti-sheep antibodies. The minimum dilution that lysis 50% of the cells determines the CH50.

Elevations in soluble Complement 5b-9 (sC5b-9) have been associated with increased activation of the alternative complement pathway. Increased complement activation may be seen with aHUS and other TMAs, and may be a sensitive early predictor of the development of transplant associated TMA. In a cohort of 67 pediatric patients who had an allogeneic HSCT, an increase in sC5b-9 concentration from baseline had 100% sensitivity and 53% specificity for TA-TMA.

The STEC-PCR assay can be confirmed by the detection of a Shiga toxin-producing Escherichia coli (STEC) in the patient’s stool sample. The test looks for the presence of ST-1 and/or ST-2 genes in stool cultures. The most common cause of HUS is STEC. STEC-HUS accounts for over 90 percent of cases of HUS in children in the US and it is one of the main causes of acute kidney injury in children under the age of three years.