False positive test results

Oxaprozin- False-positive urine immunoassay screening tests for benzodiazepines have been reported in patients taking oxaprozin. False-positive test results may be expected for several days following discontinuation of oxaprozin therapy. 

Cannabinoid test interaction - False-positive urine cannabinoid test results have been reported in uninfected volunteers who received efavirenz. False-positive test results... 

Prior to isolation and purification of analytes from hair, samples must be washed to remove externally applied drug, "dirt," and oils from the exterior of the hair to produce clean extracts and prevent false positive test results. Hair may be externally contaminated with drug following environmental exposure to drug through vapor or from contact with contaminated material such as hands, money, and clothing. 

A different form of verification bias can happen if more than one reference standard is used, and the two reference standards correspond to different manifestations of disease. This study design can produce differential verification bias. Suppose test-positive patients are verified with further testmg and test-negative patients are verified by clinical follow-up. An example is the verification of suspected appendicitis, with histopathology of the appendix versus natural history as the two forms of the reference standard. A patient is classified as having a false-positive test result if the... 

In relation to immunoassays, interference from proteins (usually antibodies) should be recognized. Ismail et aft found in a survey comprising more than 5000 TSH results that interference occurred in 0.5% of the samples, leading to incorrect results that in a majority of the cases could have changed the treatment. Rotmensch and Cole" described 12 patients in whom a diagnosis of postgestational choriocarcinoma was made based on false-positive test results for... 

Indication The test is used to discriminate patients with pheochromocytomas from patients with false-positive test. results for catecholamines or metabolites in either plasma or urine. 

Eisenhofer G, Goldstein DS, Walther MM, Friberg P, Lenders JW, Reiser HR, Pacak R. Biochemical diagnosis of pheochromocytoma How to distinguish true-from false-positive test results, J Clin Endocrinol Metab 2003 88 2656-66. 

Severe inflammation, excess amount of bacteria, crystal-luria, hematuria, poor preservation of cells, and poor cellularity (<50 cells) are some of the causes of test failure. Prior radiation therapy in patients with a history of prostate cancer, endometrial cancer, or local/systemic chemotherapy can cause persistent chromosomal aberrations that can lead to erroneous false-positive test results. One should exercise caution to obtain a proper history. Seminal vesicle cells can look highly atypical or aneuploid, which can also lead to similar false-positive results. Presence of the characteristic golden yellow pigment in the cytoplasm serves as a clue for identifying these cells. 

A report describes two patients suspected of having Cushing s syndrome because the overnight suppression test with dexamethasone 1 mg had not suppressed their cortisol levels. Further investigation found no clinical evidence of Cushing s syndrome and the false-positive test results were attributed to the fact that both patients were taking carbamazepine 400 mg three times daily at the time of the test. The test was repeated in one patient 3 weeks after carbamazepine was stopped and it indicated normal cortisol suppression. A study in 8 healthy subjects found that, in the presence of carbamazepine 800 mg daily, the dosage of dexamethasone needed to suppress cortisol secretion (as part of the dexamethasone adrenal suppression test) was increased two to fourfold. A further study found that it took 2 to 13 days for false-positive results to occur after carbamazepine was started, and 3 to 12 days to recover when the carbamazepine was stopped. ... 

The use of dimethyl sulfoxide (DMSO) as an aid to mutagenic testing of organic HPPs is to be deprecated. This technique is commonly used to render the substance being tested at least partially soluble and thus more bioavailable. In the case of organic pigments, however, any solubilization which may take place essentially transforms the pigment into a bio-available dyestuff, rather than an inert, unreac-tive, and non-bioavailable substance, and can produce false positive test results. 

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