47
Table 9. Summary of Screening Laboratory Findings and
Diagnosis of Complement Deficiencies
CH50 assay AH50 assay Possible diagnoses
NL NL Normal
NL Low Properdin defect
NL 0
Factor B
a
or factor D defect
Low NL or low Consumption likely, regulatory component (factor H,
factor I) defect possible
0 NL C1, C2, or C4 likely absent
0 0 C3 or C5-C9 likely absent
e clinical presentation is suggestive of a complement deficiency (Table 6) or
evaluation of other immune function is thus far normal, and the clinical presentation is
at least consistent with complement deficiency. Note that this table does not consider
possible lectin pathway defects. Both CH50 and AH50 results will be normal in
the setting of MBL deficiency. See the text for discussion of lectin pathway defects
and function. is algorithm can be used whether tests for the classical pathway
(CH50 assay) and alternative pathway (AH50 assay) are performed simultaneously
or sequentially. e CH50 assay is readily available in many hospital laboratories;
the AH50 (also called the AP50) assay is not. e AH50 assay is available from the
Complement Laboratory of the National Jewish Medical Center in Denver, Colorado.
Genetically determined defects in the alternative pathway leading to absent activity
in the presence of a normal CH50 result are very rare. Note also that complement
components are unstable and tend to degrade with time, especially if blood or plasma
is warmed. For the most accurate measurements, blood specimens should be placed on
ice or refrigerated aer drawing. If complement consumption is possible or suspected,
the AH50 assay might not necessarily be helpful. A convenient way available in most
hospital laboratories to test for consumption is to measure levels of factor B and C4,
reflecting activation of the alternative or classical pathway, respectively. If levels of both
of these (or other combination) are low, consumption of complement is assumed, and
a reason should be explored. Note that deficiency of factor H, factor I, or properdin
could lead to a diminished level of C3 and other components. is table has not been
constructed to evaluate all of the control proteins. In the presence of an appropriate
clinical history, low C4 levels in the presence of normal C3 levels might suggest
hereditary angioedema, and the levels and function of C1 inhibitor should be explored.
Such patients will have low CH50 results, but they will not be 0.
a
Note that homozygous deficiency of factor B has never been reported.
