Stephen Davis wrote:
>We recently encountered a case where a lady admitted for sterilisation had a >negative urine pregnancy test and underwent sterilisation. She was >subsequently shown to be pregnant with dates consistent with conception >occurring prior to the procedure.
>Should a sensitive serum test have been used rather than a urine test?
>From experience and a quick review of the literature, 1) In early pregnancy, serum hCG becomes positive a couple of days before random urine hCG, on average; 2) serum hCG is slightly higher than urine hCG, on average, probably due to more of the urine hCG being partially degraded and depending on the degree to which a particular assay detects the degraded forms. In addition, urine pregnancy tests are almost always done on random urine samples, so sometimes those samples will be relatively dilute (of course, sometimes a relatively concentrated random urine may have an hCG concentration greater than the serum concentration).
In my opinion, if the pregnancy test is being done in order to avoid potential damage to a fetus from other diagnostic or therapeutic procedures, a serum hCG should be done and caution should be used even if the serum hCG is in the borderline range. One could make an argument for doing a urine hCG in addition to the serum hCG to possibly pick up those few very early pregnancies in which the urine may be positive if the urine is relatively concentrated.
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