The Relationship of Serum Cobalamin to Methylmalonic Acid Concentrations and Clinical Variables in Cats

Background Serum cobalamin concentration [ CBL ] suggests CBL deficiency in cats but serum methylmalonic acid concentration [ MMA ] more accurately indicates CBL deficiency. Objective To examine the ability of [ CBL ] to predict CBL deficiency defined by increased [ MMA ], and relationships of [ CBL ] and [ MMA ] with select clinical and clinicopathological variables. Animals One hundred sixty‐three client‐owned cats with [ CBL ] measurements, 114 cats with simultaneous [ MMA ] measurements; 88 cats with medical information. Methods Prospectively collected [ CBL ] and [ MMA ] were compared using scatter plots, receiver operating characteristic and correlative analyses with historical [ CBL ] thresholds and those identified in the study. [ CBL ] and [ MMA ] were compared retrospectively to specific clinical and clinicopathological variables. Results [ CBL ] correlated negatively with [ MMA ] (τ = −0.334, P  < .0001). [ MMA ] ≥ 1,343 nmol/L identified CBL deficiency. [ CBL ] = 209 pg/mL optimized sensitivity (0.51), specificity (0.96), PPV (0.89), and NPV (0.74) for detecting [ MMA ] ≥ 1,343 nmol/L. Prevalence of CBL deficiency was 42% (48/114) when defined by [ MMA ] ≥ 1,343 nmol/L versus 23% (27/114) by [ CBL ] ≤ 209 pg/mL. Unexpectedly, 23 and 45% of 48 cats with [ MMA ] ≥ 1,343 nmol/L had [ CBL ] > 900 pg/mL and 290 pg/mL (historical thresholds). [ CBL ] correlated with mean corpuscular volume (τ = −0.199, P  = .013) and [MMA] with hematocrit (τ = −0.28, P  = .006). Conclusions and Clinical Importance Cobalamin deficiency ([ MMA ] ≥ 1,343 nmol/L) occurred in 42% of cats and is predicted with high specificity by [ CBL ] ≤ 209 pg/mL. CBL status correlates with microcytosis and anemia. Discordance between [ CBL ] and [ MMA ] cautions against relying on any single marker for determining CBL status.