Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (413 page)

   Microangiopathic hemolytic anemia with Hb <8 g/L.

   WBC may be increased (with neutrophilia) or normal counts.

   Platelets: severe thrombocytopenia (usually <20,000/μL) rapidly responsive to effective therapy.

   Peripheral blood smear (PBS): Fragmented red cells (schistocytes) (see p. 1121) represent >1% of RBC (or two or more per HPF); they decrease with response to therapy (schistocytes are absent in rare cases). Characteristics: nucleated red cells; basophilic stippling; polychromasia resulting from reticulocytosis.

   LDH is very elevated (>1,000 U/L at presentation is not unusual); LDH levels decrease with therapy and are useful in assessing response.

   Haptoglobin is decreased.

   Indirect bilirubin is elevated.

   Direct Coombs test is negative.

   Coagulation studies are normal and help in ruling out DIC (see p. 458).

   Increased creatinine may occur, but marked increase favors HUS.

   Serology for HIV is necessary to rule out HIV as a causative agent.

   ADAMTS13 (see Figure
9-4
).

   ADAMTS 13 measurements are most helpful retroactively to confirm the diagnosis of TTP and for follow-up because the assay provides useful prognostic information. Newer, “second-generation” assays are easier to perform, available in some commercial laboratories, and have a short turnaround time.