Medication assay interference

L-Dopa medication may interfere with the assay. Stop L-dopa at least 3 days before blood collection. 

Thus, the clinician might use TDM with bupropion to guard against the development of unusually high plasma levels of the parent drug or its metabolites. That would be particularly true for the medically compromised or the patient on other drugs that could interfere with the clearance of bupropion. In such a case, the laboratory should assay the parent drug and its three major metabolites—hydroxybupropion, threohydrobupropion, and erythrohydrobupropion. 

Substances with low solubility in acid can interfere with the DNPH assay. Mandel-amine (methenamine man delate), an antibacterial medication, and radiopaque contrast material will form a precipitate immediately upon addition of the DNPH reagent. The color and immediacy of formation of this precipitate distinguishes it from the yellowish precipitate of a-keto acid hydrazones. It is very important to have information on patient medications prior to evaluation of the DNPH test. 

Co-medications or constituents found in pathologically altered patient specimen are major causes for both ion yield fluctuations. Special measures have to be taken to reduce such effect and cause has to be taken to evaluate these accuracy limiting interferences prior to bringing an LC-MS/MS assay into the highly regulated clinical routine environment. 

Emit and FPIA assays were studied concurrently in a study of heroin users by Kintz and Mangin. The performance of the Emit assay was not satisfactory false negative results were observed in nearly 45% of the subjects studied. The problems were attributed to interference by color and turbidity of the hair extract. The results of the FPIA were acceptable, corresponding well to results obtained with GC/MS (sum of morphine and codeine concentrations). In a subsequent report, Kintz et al. further studied the FPIA with samples obtained from a medical examiner s office. In a total of 40 subjects, 14 were positive for opioids in hair by FPIA and GC/MS. The FPIA results were similar to concentrations previously reported in the literature. 

A number of medications have been shown to alter thyroid function and thyroid function tests.Few drugs are associated with the development of clinically significant thyroid disease (amiodarone may be an exception), but difficulty in the interpretation of thyroid function tests results when patients are placed on medications that affect thyroid function testing. In general, drugs do not interfere chemically with the assays for thyroid hormones or TSH. The medications most likely to affect TSH concentrations are glucocorticoids and dopamine (reduced TSH concentrations) and amiodarone (increased TSH concentrations). The most commonly encountered variations in thyroid hormone measurements induced by medications are reduced peripheral conversion of T, to T3 or altered binding of T4 and T3 to carrier proteins. Some of the medications that affect thyroid function tests are shown in Table 52-2. 

The number of samples to be reanalyzed should be sufficient to assess the performance of the assay method throughout the dynamic range, including concentrations around Cmax and in the elimination phase, for the analyte(s) of interest. A minimum of 20 samples should be selected and reanalyzed. The acceptance criteria for ISR are similar to those described in Section 122.6, that is, the analyte concentration obtained from the repeat analysis should be within 20% when compared with the initial concentration value for at least 67% of the samples selected. Large differences between the repeat and initial results may indicate analytical issues (e.g., instability, inhomogeneity, concomitant medication interference) and should be investigated. Adequate steps should be taken to minimize inaccuracy. 

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