Blood Carboxyhemoglobin Concentrations as a Diagnostic Biomarker of Hemolytic Anemias in Cats

ABSTRACT Background Endogenous carboxyhemoglobin (COHb) production is a byproduct of hemoglobin metabolism. Hypothesis Blood carboxyhemoglobin concentrations are higher in cats with hemolytic anemia (HA). Animals Twenty cats with HA, 29 cats with non‐HA, and 22 controls were prospectively followed. Methods Blood tests were performed upon admission. The Mann–Whitney and Kruskal–Wallis tests were used for comparisons. Receiver‐operating characteristic (ROC) analyses tested COHb as a marker of HA or survival. Results The HA group included 17 cats with immune‐mediated HA and 3 with Heinz body (HB) anemia. In the non‐HA group, leading diagnoses included kidney disease (n = 14), acute/chronic blood loss (n = 11) and pancytopenia (n = 3). Carboxyhemoglobin concentrations (median [IQR]) significantly differed between cats with HA (5.55% [1.9]) and cats with non‐HA (1.9% [0.7]) or controls (1.9% [0.67]; p < 0.001 for both), but not between the last two groups (p = 0.6). Among 13 nonanemic stray cats with significant HB formation, the median (IQR) COHb concentration was 6.1% (1.2). The area under the ROC curve for COHb as a predictor of HA among all anemic cats was 0.996 (95% CI, 0.985–1), with an optimal cut‐off point of 2.95% yielding a sensitivity/specificity of 95% (95% CI, 76%–99%) and 100% (95% CI, 88%–100%), respectively. Survival and COHb concentrations were not associated in either group. Conclusions and Clinical Importance COHb proved a useful ancillary test in cats with suspected HA. Nevertheless, endogenous COHb production occurs with the absorption of large hematomas, not studied herein, or during hemolysis irrespective of anemia. These caveats must be considered when applying the present findings to the clinical and research setting.