Drug testing false-negative results

The file Is used routinely In the laboratory at the National Institutes of Health In an attempt to explain abnormal test results The resident physicians affiliated with the Clinical Chemistry Service discuss the results with the patient-care physicians and determine If the results were due to the patient s clinical state or to a drug effect This close monitoring of test results has led to recognition of deficiencies In what Is believed are specific enzymatic procedures for the measurement of glucose and uric acid Likewise, the gualac procedure for occult blood In feces was found to yield false negative results under certain circumstances This has prompted the development of a more specific procedure (Jaffe et al unpublished) ... 

Measurement of plasma tryptase levels immediately (1-4 h) following an episode may be useful to confirm that a histaminoid reaction has occurred. Some four weeks later, intradermal skin tests and specific antibody tests (radioallergosorbent PAST, enzyme-linked immunosorbent ELISA) may help to identify the causative drug, although these tests do produce false-positive and false-negative results. 

Drug/Lab test interactions Diagnostic pregnancy tests based on hCG may result in false-negative or false-positive interpretations in patients on promethazine. 

Helicobacter pylori can be detected histologically from antral biopsies obtained at gastroscopy, or biochemically. In the CLO test an endoscopic biopsy specimen is incubated in a medium containing urea and an indicator which chages colour if ammonia is produced. Proton pump inhibitors and bismuth compounds suppress but do not eradicate Helicobacter pylori, and results may be falsely negative if any of these tests is carried out within a month of taking these drugs. 

---