I was having an email conversation with the director of the PKU clinic at our tertiary care children's hospital last month and the subject of false negatives came up. He said that he was not aware of ANY false negatives provided the tests were drawn after 24 hours. The cut off is >2.3 and the tests are sent to Massachsetts.

Gail Hertz, MD, IBCLC, FABM 
Author of The Little Green Book of Breastfeeding Management
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> On Dec 7, 2013, at 1:22 PM, Rachel Myr <[log in to unmask]> wrote:
> 
> When I first began practice, we needed to wait until a baby was at least 72
> hours old to obtain a blood sample for a reliable test for PKU (the main
> condition screened for in newborns). After some years, the analysis
> technique was improved and the tests were sufficiently reliable at 60 hours
> after birth.  A couple of years ago they made changes again, and now we are
> asked to obtain the blood sample as close to 48 hours after birth as
> possible.
> In the case of PKU they are looking for substances that begin to accumulate
> in the baby's blood as soon as the baby is on its own - without regard to
> how much or how little protein the baby has ingested.
> 
> I see the expressions 'false negatives' and 'false positives' have been
> used in several posts. A false negative result is when an individual who in
> fact has a given condition, tests negative for the condition. A false
> positive is when an individual who in fact does NOT have the condition,
> tests positive for it.  When reading the posts in this thread, it seemed to
> me that these ideas were reversed.
> 
> A very sensitive test will give a lot of false positive resutls (a good
> example is the doorstep fetal monitor strip - a lot of them look alarming
> but almost none of the babies are actually in any danger). A
> not-so-sensitive test will give a lot of false negative results, i.e.
> individuals who have the condition being tested for, may not be
> discovered.  When choosing screening tests, one of the things considered is
> how many false positives it will generate, requiring follow-up testing of
> heatlhy people to determine whether they are in fact at risk for anything,
> and how many false negatives, i.e. how many people with the condition in
> question will be missed by the screening procedure.  Generally we shoot for
> a test that is a little more sensitive than necessary, rather than that is
> not sensitive enough.
> 
> Rachel Myr
> Kristiansand, Norway
> 
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