Troponin I is the regulatory subunit of troponin complex associated with the actin thin filament within muscle cells. Cardiac troponin I (cTnI) is a good marker for diagnosis of myocardial damage. Several immunoassays are available for determination of cTnI in serum. The Stratus cTnI fluorometric enzyme immunoassay (Dade International) uses alkaline phosphatase (ALP) substrate. The microparticle enzyme immunoassay (MEIA) for cTnI (Abbott Laboratories) also uses ALP conjugate. On the other hand, the chemiluminescent assay (CLIA) for cTnI (Bayer Diagnostics) does not use ALP. ALP activity may frequently be elevated in serum of patients being evaluated for suspected myocardial infarction. Therefore, we studied the potential interference of ALP in cTnI assays. Serum pools were prepared from patients, and various concentrations of ALP solution were added to different aliquots. The cTnI concentrations were measured by the Stratus, MEIA, and CLIA assays. We observed no interference of ALP in the MEIA and CLIA assay for cTnI. On the other hand, we observed significant positive interference of ALP when cTnI concentrations were measured using the Stratus.

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Stability of therapeutic drugs in sera collected in Becton-Dickinson VACUTAINER serum separator SST tubes has been well studied. Recently, the Greiner Vacuette serum separator tube has become available for blood collection. However, stability of therapeutic drugs in sera when the specimen is collected in the Greiner tube has not been reported. The authors studied the stability of 15 commonly monitored drugs in sera when stored on the gel of the Greiner serum separator tubes. The drugs studied were amikacin, gentamycin, tobramycin, vancomycin, digoxin, quinidine, theophylline, carbamazepine, phenobarbital, phenytoin, valproic acid, tricyclic antidepressants, salicylate, acetaminophen, and ethanol. The authors compared the concentrations of drugs in sera stored in plain tubes (no gel) and in sera stored in the Greiner tubes containing serum separator gel. They observed a significant decline in the concentrations of tricyclic antidepressants when stored in the Greiner tubes. Interestingly, concentrations of amitriptyline declined more than its metabolite nortriptyline and concentration of imipramine also decreased more than its metabolite desipramine. The concentration of carbamazepine also decreased slightly over time when serum was stored in the Greiner tube. Although declines in carbamazepine concentrations on prolonged storage in the Greiner tubes were statistically significant, the decreases may not be clinically significant. The concentrations of the other drugs studied did not decline when stored in the Greiner tubes. The authors conclude that the Greiner brand tube is not suitable for blood collection for analysis of tricyclic antidepressants.

Chan Su, Lu-Shen-Wan, Dan Shen, and Asian ginseng are traditionally used to treat a number of conditions, including cardiovascular disease. All of these traditional Chinese medicines exhibit cardioactive properties. Digoxin is a cardioactive drug with a narrow therapeutic range (0.8-1.9 ng/mL). A patient taking digoxin may also take these Chinese medicines for their cardiotonic effects. Moreover, the active components of these medicines that are responsible for cardiotonic effects bear structural similarities to digoxin. Therefore, we studied the potential interference of these Chinese medicines with two digoxin immunoassays--the Tina-quant (Roche Diagnostics) and the Beckman (Synchron LX system)--and compared the values with the fluorescence polarization immunoassay (FPIA; Abbott Laboratories). When very small amounts (2-5 microL) of aqueous extract of Chan Su or Lu-Shen-Wan were added to drug-free serum, we observed high digoxin-like immunoreactivity with the FPIA. In contrast, when ethyl acetate extract of Dan Shen or microliter amounts of ginseng extract were added to drug-free serum, we observed modest digoxin-like immunoreactivity with the FPIA, but no apparent digoxin activity with the Roche and Beckman digoxin immunoassays. When aliquots of a digoxin pool prepared from patients receiving digoxin were supplemented with these Chinese medicines, we observed the most significant interference with the FPIA. The presence of endogenous digoxin-like immunoreactive substances can have additive effects with these Chinese medicines and falsely increase apparent digoxin levels by the FPIA. On the other hand, the Roche and Beckman assays were free from interference from DLIS but showed significant interference from Chan Su and Lu-Shen-Wan. We conclude that the FPIA showed the most significant interference from all four of the Chinese medicines we studied. However, the Roche and Beckman assays showed no interference from two (Dan Shen and Asian ginseng) of the four Chinese medicines we studied.