The effect of physiological urine dilution on pregnancy test results in complicated early pregnancies

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Abstract
This study addresses the likelihood of false negative urine pregnancy test results, due to physiological urine dilution as described in some anecdotal reports. In this prospective study 320 pregnancy tests were performed on urine samples of varying concentrations obtained from 40 women, with suspected complications of early pregnancy, who had presented for ultrasound scans. Four different pregnancy tests were used and serum βhCG levels were measured quantitatively. Despite a mean fivefold increase in urine dilution, the pregnancy tests with low βhCG detection limits maintained maximal sensitivity. The detection of βhCG in dilute urine was adversely affected by using pregnancy tests with higher βhCG detection limits and these tests should be used with caution when assessing gynaecological emergencies.