is also associated with high lactate dehydrogenase and very low haptoglobin (a surrogate marker for high free hemoglobin) serum levels. If urine is produced, urine dipstick is positive for protein and blood (mostly because of hemoglobinuria). Serum complement levels may show low or normal C3 and/or C4 levels: although routinely done, these tests are not helpful because more than 50% of patients with a confirmed diagnosis of aHUS do not have abnormal complement levels.10 Measurement of soluble C5b9 levels is a sensitive marker of activation of the terminal complement pathway,11 but it is an expensive test that is only done in specialized laboratories. CH50, which measures total complement activity, is typically used for routine monitoring of complement inhibition by eculizumab.12 Ideally, additional blood samples should be drawn before infusing blood products
or initiating blood-based therapies to ensure the reliability of tests done to identify genetic mutations, autoantibodies, or an enzymatic deficiency (see sections below).
TABLE 20.1 Key Clinical Information to Distinguish aHUS From C3G
TABLE 20.2 Atypical Hemolytic-Uremic Syndrome: Characteristics of Kidney Biopsy