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The appropriateness of serum digoxin concentration (SDC) orders was evaluated with respect to indication for use, sampling time, and action taken by physicians when the reported SDC was out of the normal therapeutic range; the effect of the two data-collection methods used (retrospective and concurrent audits) on the results was studied. Criteria for the appropriate use of SDCs were approved by the medical staff through the pharmacy and therapeutics committee. Patients on adult medicine services were entered into the study as daily SDC determinations were reported by the clinical laboratory. Most of the SDCs were evaluated using approved criteria by primary pharmacist clinicians who were concurrently monitoring drug therapy and participating with the treatment team. A retrospective audit of the same patients was conducted, using only chart review. A total of 134 SDCs involving 78 patients were evaluated. Concurrent-audit results indicated that 18.7% of the SDCs were ordered without an appropriate indication, 16.4% were sampled incorrectly with respect to proper timing, and 8.2% did not result in dosage adjustments when indicated. With respect to appropriate sampling time and overall use of SDCs, significantly more SDCs met the standards under concurrent audit than under retrospective audit. The retrospective chart review method of auditing may not detect as much pertinent information as is desirable.

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The effect of renal function and digoxin use in adult patients on interference from digoxin-like immunoreactive substances (DLIS) with three digoxin immunoassays was studied. Hospital patients entered into the study were categorized into the following groups according to renal function: group I (serum creatinine less than 1.5 mg/dL), group II (serum creatinine 1.5-2.5 mg/dL), group III (serum creatinine greater than 2.5 mg/dL, not on hemodialysis), and group IV (serum creatinine greater than 2.5 mg/dL, on maintenance hemodialysis). Medical records were reviewed to determine whether or not patients were receiving digoxin. Excess sera for analysis of serum digoxin concentrations (SDCs) was collected from routine laboratory tests. Serum samples were assayed singly by fluorescence polarization immunoassay (FPIA, Digoxin I, Abbott), radioimmunoassay (RIA, Micromedic), and affinity-column-mediated immunoassay (ACMIA, aca, E.I. du Pont). Correlation of SDCs obtained by RIA and ACMIA with FPIA results was determined using linear-regression analysis. A total of 177 patients met the study criteria; 98 were receiving digoxin. In patients on digoxin, SDCs by RIA were significantly higher than those obtained by FPIA in group II and III patients. SDCs obtained by ACMIA correlated well with and were not significantly different from those obtained by FPIA in any of the patient groups. Maximum differences and mean absolute differences in SDCs obtained by RIA were greater than those for ACMIA when compared with FPIA values in all patient groups. Over 40% of patients with renal dysfunction not on digoxin had false-positive SDCs by RIA; the highest of these values was seen in groups II and III.(ABSTRACT TRUNCATED AT 250 WORDS)

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Enzyme multiplied immunoassay (Emit) commonly is used to determine aminoglycoside concentrations. Its accuracy generally is comparable to that of radioimmunoassay (RIA). Poor reproducibility and questionable quantitation by the Emit assay have been reported when heparinized, severely lipemic, or icteric samples have been used. However, the significance of these interferences is documented poorly. We observed a ESRD patient in whom the underestimation by Emit of the tobramycin concentration in a plasma sample (heparin concentration of 41 U/ml) could have resulted in excessive drug administration and potential toxicity. Tobramycin therapy was initiated with a loading dose of 1.6 mg/kg and three tobramycin concentrations were obtained (2, 12, and 36 hours post-infusion) to define the patient's pharmacokinetic parameters. The first and third samples were collected in serum specimen tubes while the second sample was collected in a plasma tube. The tobramycin concentration in the plasma sample measured by Emit was 82 percent lower than the RIA value. Analyses of other samples by both procedures revealed no clinically significant differences. This case demonstrates that the presence of heparin may interfere with the Emit tobramycin assay in the clinical setting. The degree of reduction in tobramycin concentrations may be dramatic and potentially can alter a patients apparent tobramycin dosing requirements. Further investigation is warranted.

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