Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1074 page)

   Limitations

   Immunoassay-based tests (EMIT, FPIA) for serum/plasma

   Serum must be collected in tubes without serum separator gel.

   Cells must be separated as soon as possible after collection.

   Heparinized, EDTA, and fluoridated collection tubes for plasma are acceptable.

   This assay measures the total (protein bound and free) levels of methotrexate in serum and plasma.

   It is known that aminopterin and APA (a metabolite of methotrexate) cross-react significantly with the EMIT assay and less so with the FPIA.

   Patients should be rebaselined when changing the methodology.

   Samples are stable for up to 24 hours when refrigerated and protected from light..

METHYLMALONIC ACID

   Definition

   Methylmalonic acid (MMA) is an intermediate in the propionate degradation pathway. Deficient activity of the enzyme responsible for the conversion of methylmalonyl CoA to succinyl CoA (methylmalonyl CoA mutase) results in the organic aciduria known as methylmalonic aciduria, with a classical presentation of neonatal onset metabolic acidosis, hyperammonemia, and poor outcome if untreated. The concentrations of the metabolic markers MMA and homocysteine (Hcy) are considered to be more sensitive indicators of vitamin B
₁₂
status. Both MMA and Hcy increase in vitamin B
₁₂
deficiency. However, Hcy has been shown to have low specificity, being influenced by lifestyle factors such as smoking and alcohol intake and increasing in patients with folate deficiency and renal impairment.

   
Normal range:
0.00–0.40 μmol/L.

   Use