25125 Complement, Alternate Pathway, Functional (AH50)

This assay is a functional test and is dependent on correct sampling, storage, and shipping conditions. Both degradation by temperature and consumption of complement components will lead to false low function results. These are difficult to differentiate from real complement dysregulation.

While preanalytic handling can lead to false-positive results, it is far less likely that it would lead to false-normal results. If more than one component is measured as low, it is important to look for technical errors.

Complement testing may be ordered in several circumstances where standard treatment includes plasmapheresis or plasma exchange. The procedure itself, if traumatic, may activate complement so may not reflect what is going on with the patient's complement system. The recommendation is to collect blood prior to the plasma exchange whenever possible.

Functional results inconsistent with the clinical history should be verified with a new blood draw.

Specimens should be frozen immediately after collection.

Long term stability is optimal when the sample is kept at -70 degrees Celsius or lower prior to testing.

Complement proteins are components of the innate immune system. There are 3 pathways to complement activation: (1) the classical pathway, (2) the alternative (or properdin) pathway, and (3) the lectin (or mannose-binding lectin, MBL) pathway.

The total complement (CH50) assay (COM / Complement, Total, Serum) assesses the classical complement pathway including early components that activate the pathway in response to immune complexes (C1q, C2 and C4), as well as the terminal complement  components (C3, C5, C6, C7, C8, C9) involved in the formation of the membrane attack complex (MAC). The CH50 assay will be abnormal if there are specific hereditary or acquired C1-C9 complement component deficiencies or if there is consumption of complement due to immune (or autoimmune) complexes.

This assay is a screening test for complement abnormalities in the alternative pathway. The alternative complement (AH50) pathway shares C3 and C5-C9 components but has unique early complement components designated factors D, B, and properdin, as well as control proteins factor H and factor I. This pathway can be activated by spontaneous hydrolysis of C3 or by microbial polysaccharides and does not require immune complex formation. Patients with disseminated infections with pyogenic bacteria in the presence of a normal CH50 may have a decreased AH50 due to hereditary or acquired deficiencies of the alternative pathway. Patients with deficiencies in the alternative pathway factors (D, B, properdin, H, and I) or late complement components (C3, C5-C9) are highly susceptible to recurrent Neisserial meningitis. The use of the CH50 and AH50 assays allow identification of the specific pathway abnormality.

Functional testing for complement pathways activity is indicated in the study of complement components deficiency, where testing serves as a first-tier screening, or in the study of complement dysregulation. Complement dysregulation is a general grouping of complement conditions where there is loss of control of the complement cascade with over-activation. In several cases, the complement system will attack the host and the over-activation of the complement cascade may cause disease.

Over-activation of the alternative pathway usually presents with renal function impairment, in rare conditions such as atypical hemolytic uremic syndrome (aHUS) and C3 glomerulopathies (dense deposit disease [DDD] and C3 glomerulonephritis [C3GN]).

The use of complement inhibitor therapies such as eculizumab and ravulizumab will result in the blocking of C5. C5 is necessary for the AH50 test to progress until the formation of the MAC. Hence, in the presence of eculizumab or ravulizumab, AH50 results will be decreased or undetectable