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Giant cell arteritis — ABIM Board MCQ

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HardRheumatology/Allergy/ImmunologyGiant cell arteritisABIM Board

A 71-year-old has cold-induced acrocyanosis, red-cell agglutination on the smear, elevated LDH, low haptoglobin, and indirect hyperbilirubinemia. Which laboratory pattern most strongly supports primary cold agglutinin disease?

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Correct answer: C — Direct antiglobulin test strongly positive for C3d with a significant cold-agglutinin titer

The best answer is “Direct antiglobulin test strongly positive for C3d with a significant cold-agglutinin titer”. Cold agglutinin disease is usually mediated by a cold-reactive IgM antibody that fixes complement. Red cells are therefore strongly C3d positive on a monospecific direct antiglobulin test and usually negative or only weakly positive for IgG; a clinically significant cold-agglutinin titer and thermal amplitude support the diagnosis. Warm IgG hemolysis, acquired hemophilia, and TTP have different laboratory signatures.

Reference: Cold agglutinin disease review: https://pmc.ncbi.nlm.nih.gov/articles/PMC6497508/