Serum IgM Concentrations in Normal, Fit Horses and Horses with Lymphoma or Other Medical Conditions

The purposes of this study were to (1) prospectively establish serum IgM and IgG concentrations in normal, fit, adult horses over time and (2) determine the accuracy of serum IgM concentrations for diagnosing lymphoma. Serial IgM and IgG concentrations were measured with a radial immunodiffusion assay in 25 regularly exercised horses at 6-week intervals. Horses had serum IgM concentrations ranging from 50 to 242 mg/dL over 5 months, with 20% of horses having IgM less than or equal to 60 mg/dL. The normal range for IgM in fit horses should be considered 103 +/- 40 mg/dL and a cut-point for an IgM deficiency, less than or equal to23 mg/dL. IgG concentrations ranged from 1,372 to 3,032 mg/dL. Retrospectively, medical records of adult horses (n = 103) admitted to the Cornell University Hospital for Animals for which serum IgM was measured were examined. Horses were categorized as "lymphoma negative" (n = 34) or "lymphoma positive" (n = 18). The sensitivity and specificity of a serum IgM concentration (less than or equal to60 mg/dL) for detecting equine lymphoma was 50 and 35%, respectively. At the new cut-point (less than or equal to23 mg/dL), the sensitivity was low at 28% and the specificity improved to 88%. The negative predictive values at various population prevalences indicate that a horse with a high serum IgM (>23 mg/dL) is unlikely to have lymphoma, whereas the positive predictive value (70%) does not allow for reliable determination of lymphoma in a horse with serum IgM less than or equal to 23 mg/dL. Therefore, serum IgM concentrations should not be used as a screening test for equine lymphoma.