A BetterMethodfor Eliminating ... - Clinical Chemistry

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metabolized to sulfapyridine and 5- aminosalicylic acid. Little of the 5- aminosalicylic acid is absorbed; most is excreted unchanged. Nearly all the sulfapyridine,.
it3’ of 55 U/L). The diagnostic efficiencies increase to 92% at 12 h and 95% or more at 24 and 48 h when a cutoff value of65 U ofLD-1 activity per liter is used. This study confirms the reported early occurrence of an increased LD-1 within the first 24 h after infarct (2,3) with a 90% efficiency in the first 16 h

(3). We also point out the high error rate that may be expected if the LD-1 activity is measured earlier than 12 h after admission. Our results indicate that LD-1 assay is most efficient if a cutoff value can be assigned for an optimum sampling time. However, a measurement of LD-1 activity at the time of admission may be indicated if the total LD activity is above normal.

The use of a bivariate distribution plot of total LD activity vs percent LD-1 as a means to identify individuals with MI with greater accuracy is similar to the LD-1 activity vs LD-1/LD activity ratio recently reported (7).

References 1. Usategui-Gomez

M, Wicks

RW, War-

shaw M. Immunochemical determination of the heart isoenzyme of lactate dehydrogenase (LD-1) in human serum. Clin Chem 25, 729-734 (1979). 2. Liu F, Belding R, Usategui-Gomez M, Reynoso G. Immunochemical determination of LD-1. Am J Clin Pathol 75, 701-707

(1981). 3. Bruns

DE, Emerson JC, Intemann S, et

al.

Lactate dehydrogenase isoenzyme-1: Changes during the first day after myocar.

dial infarction.

Clin Chem 27, 1821-1823 (1981). 4. L.eung FY, Henderson AR. Thin-layer

agaroseelectrophoresis of lactate dehydrogenase isoenzymes in serum: A note on the method of reporting and on the lactate dehydrogenase isoenzyme-1/isoenzyme-2 ratio in acute myocardial infarction. Clin Chem 25, 209-211 (1979). 5. Vasudevan G, Mercer DW, Varat MA. Lactic dehydrogenase isoenzyme determination in the diagnosis of acute myocardial infarction. Circulation 57, 1055-1057

(1978). 6. Gordesky SE, Winsten S. LD-1/LD ratio as a diagnostic determinant for myocardial infarction. Clin Chem 28, 1239-1240

A BetterMethodfor Eliminating Salicylateinterferencewith Measurementof Acetaminophen To the Editor: Acetaminophen is available in many proprietary combinations with aspirin (acetylsalicylicacid) and salicylamides (1, 2), so any method for acetaminophen measurement should be free from interference by aspirin and its related compounds. Reed et al. (3) recently reported that a commercial kit method

for acetaminophen analysis (“Rapid Stat Kit”; Lancer Division of Sherwood Medical, St. Louis, MO 63101) suffers from a significantly stronger positive interference by salicylate than was indicated

in the package

Salicylamide, the drug most cornmonly combined with acetaminophen in “aspirin-free” analgesics, is readily hydrolyzed to salicylate. Both salicylamide and salicylate interfere positively with nitration methods (4). A direct acidlferric reduction method for acetaminophen that is free of interferences by salicylate and salicylamides was reported earlier by Liu and Oka (5). In this method a complex of ferric

2,3,6-tris(2-pyridyl)-S-triazine

have a phenolic hydrogen to donate as an active reducing agent Table 1. The potential reducing hydrogens of the phenolic groups of salicylate and salicylamides are both stabilized by internal hydrogen bonding to the oxygens of the carboxyl and amide groups, respec-

tively. These stabilizations explain the minimal interference observed with this method for acetaminophen (Table 1). In the therapeutic concentration

Table 1. Effect of Salicylate, Salicylamide, and Aspirin on Measurement of Acetaminophen by the Direct Acid/Ferric Reduction Method

7. Wang TY, Godfrey JH, Graham LG, et al. Clinical evaluation of immunochemical assay of lactate dehydrogenase isoenzyme 1 in early detection of acute myocardial infarction. Clin Chem 28, 2152-2154 (1982).