Three new tools to diagnose B12 deficiency: eGFR-adjusted methylmalonic acid (MMA₁₀₀), a bivariate reference area for MMA₁₀₀ and cobalamin, and a cobalamin deficiency index

Vitamin B12 (s-cobalamin) and methylmalonic acid (s-MMA) are often interpreted together to diagnose cobalamin deficiency, in context with patient symptomatology, which is many cases is unspecific. Today, clinicians assess test results in relation to the univariate reference limits or decision limits. As s-MMA depends on renal function (glomerular filtration rate, GFR), interpretation can be complicated. To ease the interpretation of the two measurements, we propose three new tools: First, we developed a new formula for adjusting s-MMA to an eGFR of 100 mL/min/1.73 m² (s-MMA₁₀₀). The formula was s-MMA₁₀₀ = [(eGFR/100)^0.549] × s-MMA. It was derived from the median relationship between s-MMA and estimated GFR (eGFR) in an ambulant patient population of 4342 individuals, where eGFR was calculated according to the EKFC equations. S-MMA₁₀₀ was not associated with eGFR in a US test population of 6852 individuals. Second, we constructed a combined reference range for s-MMA₁₀₀ and s-cobalamin from data in a healthy reference population (<i>n</i> = 495 individuals). Third, we proposed a new cobalamin deficiency index, CDI = s-cobalamin/s-MMA₁₀₀, and studied the effect of different decision limits on the prevalence of positive test results in a patient population. Using the 2.5 percentile of CDI in the reference population as a decision limit gave a prevalence of 5.2% positive test results in the patient population. However, as a gold standard for cobalamin deficiency does not exist, we were unable to study the diagnostic accuracy of the CDI. Therefore, the true diagnostic accuracy of these tools is yet unknown and should be investigated.