We have seen quite serious false negatives on at least one manufacturer's
product, we assume due to hook effect on the assay.
Certainly one of the manufacturers recognises that certain samples can give
false negatives, and even suggest a "repeat at 1/10 dilution if negative and
a strong possibility of pregnancy". We no longer use that particular product
as we found that doing all the negatives twice was somewhat laborious.
with best wishes
Richard
Richard Mainwaring-Burton
Consultant Biochemist
Queen Mary's Hospital
Sidcup, Kent
020-8308-3084
-----Original Message-----
From: Wills Philip (Queen Elizabeth Hospital NHS Trust)
[mailto:[log in to unmask]]
Sent: 20 December 2005 17:14
To: [log in to unmask]
Subject: ?False negative urine pregnancy results/Christmas Quiz
I would be grateful for possible explanations for the two
following cases of alleged false negative urine pregnancy
testing results using a well known kit with cut-off 25 U/l.
Please assume there is nothing wrong with the batch. All
obvious actions are underway and the company has been
informed. The kit performs perfectly well in EQA although
this particular batch was not used. Staff in A/E have had
training updated although there is no indication that the
test was not performed correctly.
CASE ONE
Came to A/E with abdominal pain on 9/11/05 and also 12/11/05.
Urine Pregnancy Test in A&E negative on both visits.
Came to A&E again 15/11/05 with abdominal pain
"Just finished period" - 13/11/05(? date of bleed prior to
that.)
Doctor performed a Urine Pregnancy Test approx 08.00h on
15/11/05 (despite 2 recent negatives) which was positive.
Serum beta HCG taken 10.34h on 15/11/05 was 2650 U/l.
The patient had an ectopic pregnancy, which had been missed
on the first 2 occasions. Patient is now doing well.
CASE TWO
Presented 15/11/05. Known to be 8 weeks pregnant. Presented
with hyperemesis, but Urine Pregnancy Test at triage, approx
15.45 was negative. Doctor repeated Urine Pregnancy Test
himself on same sample - again negative. Then asked patient
to provide another sample, which he tested himself, which was
also negative. That is 3 negative tests on 2 samples. Doctor
sent serum beta HCG, approx 16.30h on 15/11/05. The result
was greater than 10000 U/L
No adverse outcome in either case.
PS I have just checked 20 of the "suspect" cassettes against
20 consecutive ex EQA material stored at -20 C and got 100%
correct answers.
Many thanks,Phil Wills
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