Calling the kits urinary pregnancy tests is a misnomer, they should be
called "tests for the detection of urinary hCG" Detection of urine hCG does
not automatically mean that the patient is pregnant, and the result should
be looked at in the light of the full clinical picture, and confirmed by
serum hCG and Ultrasound if there is any doubt.
A typical 25u/l sensitivity urine hCG kits may give positive results at
levels as low as 6u/l depending upon the batch.
In our experience there is also a significant amount of operator error,
despite having instrument read, rather than visually read tests, which we
stopped doing because of the number of errors.
I would agree that serum hCG would be a better screening test, but certainly
our A&E is reluctant to wait for 1hr for a result. We are currently
investigating a patient who appears to have a high level of urine hCG, (by 3
different methods) but no detectable serum hCG, which one is correct remains
to be seen, but she is not pregnant after 3 months by ultrasound.
There is also a strong rumour that the waiting times in A&E are now so short
that teenage girls are coming in to A&E complaining of abdominal pain, in
order to get a free "pregnancy" test!
-----Original Message-----
From: James J Miller [mailto:[log in to unmask]]
Sent: 01 April 2005 13:45
To: [log in to unmask]
Subject: False urinary pregnancy tests
Helen Grimes commented "...it is a well known fact that urinary
pregnancy tests kits can give false positives and negatives, and there
is the problem of a dilute random urine, so since one can quantitatively
measure serum hCG, this should be the test of choice."
I agree that serum hCG is the test of choice for ruling out pregnancy
before giving potentially fetus damaging radiation or drugs, mainly
because it is more sensitive than urine hCG, but I was unaware of false
positive and false negative urine hCG (other than the lower sensitivity
and dilute urine issues). What are some other causes of FP & FN urine
hCG? References?
Thanks. -Jim
James J Miller, Ph.D., DABCC, FACB
Associate Professor, Pathology & Laboratory Medicine
University of Louisville, Louisville, KY 40292
Director of Clinical Chemistry & Toxicology
University of Louisville Hospital Laboratory
[log in to unmask]
Office: 502-852-1179
Pager: 502-478-1219
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