Semi-automated reflectance readings of urinalysis dipsticks showed better precision than visual readings in the case of artificially prepared urine samples containing glucose, ketone, and protein. Actual pathological specimens containing glucose, protein, and nitrite also showed that instrumental readings were better than visual. Results of repeated visual readings spread over three different color blocks for certain concentrations of glucose, ketone, and protein, whereas repeated instrumental readings were never spread greater than two color blocks. Subjectivity can be reduced by semi-automated dipstick urinalysis.

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Amoxicillin has been causally linked in the lay and medical literature to false-positive urine drug screens for cocaine metabolites. An exhaustive search of the peer-reviewed medical literature revealed no data to support this link. We hypothesized that amoxicillin does not cause false-positive urine drug screens for cocaine metabolites. To test this hypothesis, we examined the urine of 33 subjects administered a course of amoxicillin, subjecting the specimens to four common urine screening immunoassays. Thirty-one specimens were negative for the cocaine metabolite, benzoylecgonine (BE), by all four screening methods; two were positive for BE by all four screening methods. Both positive specimens were confirmed by gas chromatography-mass spectrometry (GC-MS) for the presence of BE at > 150 ng/mL. Three specimens that screened negative, but produced absorbance values that were intermediate between negative and positive controls, were submitted for GC-MS analysis; BE was detected in all three specimens at concentrations of 54, 94, and 119 ng/mL. Twenty-eight specimens produced screening results indistinguishable from negative controls. Within the limitations of the study design, we conclude that amoxicillin is unlikely to produce false-positive urine screens for cocaine metabolites.

Reproducibility of reading "N-Multistix" dipsticks by a semi-automated urinalysis instrument (Ames' "Clini-Tek") has been described for artifically prepared samples. Glucose, ketone, urobilinogen, and nitrite showed high reproducibility (greater than 90%) for reading multiple samples at predetermined analyte concentrations. Determination of proteinuria showed the lowest proportion of false positives (2-3%) and false negatives (0%). Determination of hemoglobinuria and bilirubinuria by dipsticks were the least reproducible. Urobilinogen showed no interference from bilirubin in concentrations up to 32 mg/liter. Precision was high for results for quality-control capsules provided by the manufacturer.

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