Packaging may lead to false positive results

BMJ 1996; 313 doi: https://doi.org/10.1136/bmj.313.7063.1010 (Published 19 October 1996) Cite this as: BMJ 1996;313:1010
  1. Adrian Edwards, Clinical fellow,
  2. Stephen Granier, Registrar in general practice,
  3. Judith Van Der Voort, Paediatric registrar
  1. Department of General Practice, University of Wales College of Medicine, Llanedeyrn Health Centre, Llanedeyrn, Cardiff CF3 7PN
  2. Department of Child Health, Cardiff Royal Infirmary, Cardiff CF2 1SZ

    EDITOR,—J H Beer and colleagues report false positive results for leucocyte esterase on urine analysis in patients concurrently using certain antibiotics.1 We have encountered false positive results for leucocyte esterase and nitrite on urine analysis.

    With Boehringer Mannheim, we are developing a kit for parents to use to test their child's urine for infection after they present their sick infant to primary care. By overcoming some difficulties encountered in primary care (for example, time constraints, availability of equipment)2 this test should help general practitioners to investigate for urinary tract infection. Initial results indicate that the test is acceptable to parents, but repackaging the sticks for the kit (in polyethylene with silica gel desiccant) has led to results that are false positive when compared with negative results of microscopy carried out in the laboratory. Tests for nitrite gave positive results in all nine samples studied, and tests for leucocyte esterase gave positive results in six (at 10–25 cells/μl by semiquantitative assay). In one sample a test for leucocyte esterase gave a result indicating 75 cells/μl but microscopy showed 0–5 white cells per high powered field. In contrast, sticks from the manufacturer's original packaging correctly identified 12 sterile urine samples as being negative for both nitrite and leucocyte esterase in the children's outpatient department.

    Thus our pilot trial indicated that the packaging led to false positive results. This is probably not insurmountable, and the problem is not as critical as false negative results would be. Clinical efficacy requires a high sensitivity to exclude urinary tract infection in children presenting with often nonspecific symptoms and to obviate further microbiological investigation. If the specificity was also high, a positive result on urine analysis would enable a presumptive diagnosis of urinary tract infection when the presentation was unclear.

    The clinical context of such tests, and the prior probability of developing urinary tract infection, is also important.3 A high clinical probability means that a negative result of a dipstick test cannot exclude the diagnosis, as in the febrile leucopenic patients described by Beer and colleagues, and microscopy and culture of urine are mandatory.4 When the clinical presentation is undifferentiated or occurs in an area of low prevalence, as is often the case in primary care, then urine analysis may help in distinguishing urinary tract infection from other diagnoses5 and identifying those patients who need urine microscopy and culture.


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