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If the patient has been to the toilet just prior to being seen by the doctor who then wishes an x-ray examination, she will be given a large glass of water to drink to expedite the production of the next specimen of urine.  The resulting dilute urine could give a negative pregnancy test even though the patient is "just" pregnant.  The recently implanted foetus is the very one which shouldn't be subjected to radiation.  Under these circumstances, serum hCG is the better option.

Elliott Simpson

-----Original Message-----
From: Trevor Gray [mailto:[log in to unmask]]
Sent: 18 June 2002 19:44
To: [log in to unmask]
Subject: Re: Negative serum hCG and radiological investigations.

In message <40CFD5CFAF2AD611A26600508B2C6AC23FBC3C@UCHG-MAIL>, Grimes,
Helen, UCHG <[log in to unmask]> writes
>We use the Hybritech serum ICON hCG assay. A negative value represents <10
>U/L. A radiographer quoted the 10 day rule, that if there was a  LMP in the
>last 10 days, then it is OK to presume the patient is not pregnant, but
>wondered as to what we claim by a negative test when the LMP is uncertain,
>and does one always accept the patient's information. Does anyone have an
>agreed protocol with regard to radiological investigations in such
>situations?
>
We are just discussing a related problem.  Our two hospitals recently
joined in unholy matrimony have two different procedures when LMP is
uncertain.  We use a serum test (sensitivity 2 U/L) but the other uses a
urine test (sensitivity unclear but probably 25 U/L).  I have put
forward that the serum test provides a  more certain cut-off and must
become positive by a few days before the urine test.  However the
obstetricians and gynaecologists disagree and have sanctioned the urine
test despite the known problems of urine sampling.  Who is right, or is
the time difference so marginal that it doesn't matter?

I have to confess that this has only recently blown up and I haven't yet
got down to the literature.  If anyone can save me some time on this I
would be grateful.
Trevor
--
Trevor Gray
Dept. of Clinical Chemistry,
Northern General Hospital,
Sheffield S5 7AU

0114 271 4309

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