False-positive urine pregnancy test due to leukocyte interfe
Published in the Annals of Laboratory Medicine, the authors present a case of a 40-yr-old woman (gravida 0, para 0) who presented to due to a survey on artificial reproductive technique (ART). She had been previously diagnosed with endometriosis and underwent laparoscopy surgery for endometriosis last year. However, her symptoms of dysmenorrhea and hypermenorrhea still persisted.

A urine pregnancy test was ordered by the clinician, and the test indicated the patient was pregnant. A second urine pregnancy test was conducted, and again, the test results came back positive. However, the result was inconsistent with the patient's actual pregnancy status.

According to the manufacturer's instructions, if the urine appears turbid, it should be centrifuged, filtered, or allowed to stand for a while before testing in order to avoid false outcomes. In this case, the patient's urine sample was judged as clear by visual determination, and the urine hCG one-step pregnancy test was performed following the instructions of the manufacturer without centrifugation. A positive result was obtained. However, the total serum hCG was detected simultaneously, indicating a concentration of 1.32 mIU/mL, which was inconsistent with the results of the urine pregnancy test.

In order to rule out the hook effect from the serum sample test, quantitative detection of hCG was performed again, using a dilution model. The outcome of this repeat examination was similar to that of the original.

In order to investigate the cause of this false-positive result, it was assumed that the interfering factors might have originated from either the content of the urine or the test kit batch.

First, the contents of the urine specimen were examined. Sediment analysis by microscopy revealed that the concentration of leukocytes and epithelial cells were 717/µL and 31/µL, respectively. With no erythrocytes present in the urine, the appearance of the patient's urine seemed clear.

Therefore, a was designed that was able to demonstrate whether cells in the urine could interrupt the performance of the urine hCG one-step pregnancy test. The urine sample was serially diluted with normal urine from a male to a WBC concentration of 600/µL, 500/µL, 400/µL, 300/µL, 200/µL, and 100/µL. These diluted samples were subjected to the urine hCG one-step pregnancy test, and the results indicated that positive test results occurred in samples having 400 or more cells per µL, while samples with less cells tested negative.

Another possible interference was a problem with the test kit batch, since it was also found that different batches of test kits gave different outcomes for the same original urine sample. In this examination, a concentration of 500 or more cells per µL gave a positive result, while the other dilutions tested negative. After centrifugation, all samples revealed a consistently negative result.

Lessons learnt:-
- The interfering factor could be the presence of cells in the urine sample or the inconsistency of the manufactured device.

- In both cases, false-positive results can be avoided by centrifugation. Although the package insert states that centrifugation, filtration, or allowing the sample to stand for a while is only required when the sample appears to be turbid, the determination of turbidity may not be consistent between medical technologists due to variations in eyesight.

Read more here: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3289784/
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