Sorted By Test Name - Mayo Medical Laboratories

MTX
8721
FMETX
91822
phenazopyridine, sulfonamides, local anesthetics, dapsone, or following ingestion of nitrites or nitrates.
Congenital methemoglobinemias are rare. They are due either to: -A deficiency of methemoglobin
reductase (also called cytochrome B5 reductase or diaphorase) in erythrocytes, an autosomal recessive
disorder -One of several intrinsic structural disorders of hemoglobin, called methemoglobin-M, all of
which are inherited in the autosomal dominant mode Sulfhemoglobin: Sulfhemoglobinemia often
accompanies methemoglobinemia. Sulfhemoglobinemia can be due to exposure to trinitrotoluene and/or
zinc ethylene bisdithiocarbamate (a fungicide). The formation of sulfhemoglobin cannot be reversed and
there is no therapy for sulfhemoglobinemia. Because patients with sulfhemoglobinemia also often have
methemoglobinemia, therapy is directed at reversing the methemoglobinemia present. Symptoms of both
methemoglobinemia and sulfhemoglobinemia are caused by anoxia and are characterized by cyanosis.
Useful For: Diagnosis of methemoglobinemia and sulfhemoglobinemia Differentiation of
methemoglobinemia and sulfhemoglobinemia from other causes of cyanosis (eg, congenital heart disease)
Interpretation: In congenital methemoglobinemia, the methemoglobin concentration in blood is about
15% to 20% of total hemoglobin. Such patients are mildly cyanotic and asymptomatic. In acquired (toxic)
methemoglobinemia, the concentration may be much higher. Symptoms may be severe when
methemoglobin is >40% of hemoglobin. Very high concentrations may be fatal. This is a consultative
evaluation in which the history and previous laboratory values are reviewed by a hematologist who is an
expert on these disorders. Appropriate tests are performed and an interpretive report is issued.
Reference Values:
Definitive results and an interpretive report will be provided.
Clinical References: Beutler E: Methemoglobinemia and sulfhemoglobinemia. In Hematology. 5th
edition. Edited by E Beutler, MA Lichtman, BS Caller, TJ Kipps. New York, McGraw-Hill Book
Company, 1995, pp 654-663
Methotrexate, Serum
Clinical Information: Methotrexate is an antineoplastic agent that inhibits the enzyme, dihydrofolate
reductase. Such inhibition causes a stoppage of synthesis of tetrahydrofolate that, in turn, results in an
inhibition of nucleotide and ultimately of DNA synthesis. Methotrexate is effective against malignancies
with rapid cell proliferation such as acute lymphoblastic leukemia, choriocarcinoma, trophoblastic tumors
in women, and carcinomas of the breast, tongue, pharynx, and testis. When used as an antineoplastic agent
against these tumors, it is administered at high dose (2.5 mg/kg or 3.3 mg/kg), and usually is followed by
leucovorin (folinic acid) rescue to salvage nontumor cells.
Useful For: Following therapy, serum concentration is used to judge whether the drug is being cleared
appropriately and verify that a nontoxic level has been attained.
Interpretation: Following a 4 to 6 hour intravenous infusion of methotrexate, postinfusion
concentrations greater than the following indicate an increased risk of toxicity if conventional low dose
leucovorin rescue is given: -24-hour postinfusion concentration: 5.0 mcmol/L to 10.0 mcmol/L -48-hour
postinfusion concentration: 0.5 mcmol/L to 1.0 mcmol/L -72-hour postinfusion concentration: 0.1
mcmol/L
Reference Values:
Nontoxic drug concentration after 72 hours: