The ratio of the concentrations of myoglobin over FABP in plasma from patients with muscle injury reflects the ratio found in the affected tissue. Since this ratio is different between heart (4.5) and skeletal muscle (20 to 70), its assessment in plasma allows the discrimination between myocardial and skeletal muscle injury in humans.

SUMMARY. To allow a more rapid determination of heart-type fatty acid-binding protein (FABP) concentration in plasma a direct non-competitive (sandwich-type) ELISA was developed which uses high-affinity monoclonal antibodies to FABP. Total performance time of the one-step immunoassay is 45 min. The standard curve was linear between 0·2-6/lg/L, and the within-run and between-run coefficients of variations were below 6 and II %, respectively. The serum FABP concentration measured in 79 healthy individuals was 1·6 (0'8) [mean (SD), range 0,3-5'0] /lg/L. The assay can be used for rapid plasma or serum FABP measurement in the early diagnosis of acute myocardial infarction. Additional key phrases: heart-type fatty acid-binding protein; monoclonal antibodiesFatty acid-binding protein (FABP) is a recently introduced plasma marker of acute myocardial infarction (AMI) in man.' 4 The plasma kinetics of F ABP (15 kDa) closely resemble those of myoglobin (18 kDa) in that significantly elevated plasma concentrations are found within 3 h after AMI which generally return to normal values within 12 to 24h. 5 7 These features make FABP a useful biochemical marker especially for the early assessment or exclusion of AMI,5.6 and for the monitoring of a recurrent infarction." Since FABP released from the heart after AMI is quantitatively recovered in plasma, FABP can also be used to estimate infarct size."As with myoglobin, small quantities of (hearttype) FABP are also found in skeletal muscle, which are released into the circulation following injury." However, as the ratio of the tissue concentrations of myoglobin over FABP is different in heart (4-5) and skeletal muscles (2G--70), the assessment of this ratio in plasma allows the discrimination between myocardial and skeletal muscle injury."The clinical application of FABP to confirm or exclude a diagnosis of AMI soon after admission Correspondence: Jan F C Glatz PhD. requires a rapid test system, which is not yet available. Because FABP does not exhibit enzymatic activity, its plasma concentration has to be measured immunochemically. However, reported immunochemical assays for FABP take about 2 to 5 h to complete. 2,3,5,9 Here we describe a one-step sandwich-ELISA for FABP in plasma which uses high-affinity monoclonal antibodies, and shows a high sensitivity with a total performance time of only 45 min. MATERIALS AND METHODSCollection of blood samples For determination of the reference range of F ABP in serum and plasma, blood samples were withdrawn from 79 healthy subjects (61 men, median age 35, range 2G--51 years; and 18 women, median age 38, range 23-51 years). Subjects were taking no medication. Blood samples were obtained between 9 and 12 am and collected in glass tubes (preparation of serum). For IO subjects, a second blood sample was collected in glass tubes containing dry heparin (preparation of plasma). After centrifugation at 1500g for IOmin, serum or plasma was collected and stored at -70°C until analysis. 263

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Using individually estimated clearance rates, renal insufficiency no longer hampers calculation of infarct size from FABP and MYO, and reliable estimates of total myocardial damage can be obtained within 24 h after first symptoms.