The correct answer is:

serum cryoglobulin testing

Educational Objective:

Examine the link between hepatitis C virus infection and cryoglobulinemic vasculitis.

Key Point:

HCV infection is associated with a mixed cryoglobulinemic syndrome, which should be suspected among infected patients who present with palpable purpura, arthralgias, and renal insufficiency.

Explanation:

Patients with long-term HCV infection are at risk for developing mixed cryoglobulinemia syndrome. Cryoglobulinemic vasculitis may present with a single manifestation or a combination of palpable purpura, Raynaud phenomenon, arthralgias or arthritis, kidney disease, and peripheral neuropathy. Patients with cryoglobulinemia have hypocomplementemia, typically with very low C4 levels, and positive rheumatoid factor.

Testing for serum cryoglobulins is diagnostic. Importantly, cryoglobulins are tested by drawing blood in tubes that have been prewarmed to 37°C (98.6°F) without anticoagulants; false-negative results are possible without this preparatory step.

A kidney biopsy and electromyography will show glomerulonephritis and peripheral neuropathy, but neither will confirm the diagnosis. A liver biopsy would only confirm HCV infection.

References:

Arntfield RT, Hicks CM. Systemic lupus erythematosus and the vasculitides (Chapter 108). In: Walls R, et al. Rosen’s Emergency Medicine: Concepts and Clinical Practice. 9th ed., 2018:1402-1417.

Himoto T, Masaki T. Extrahepatic manifestations and autoantibodies in patients with hepatitis C virus infection. Clin Dev Immunol. 2012;2012:871401.

Lauletta G, Russi S, Conteduca V, Sansonno L. Hepatitis C virus infection and mixed cryoglobulinemia. Clin Dev Immunol. 2012;2012:502156.