Nikki Lee <[log in to unmask]> wrote:

> Main outcome measures Results of serological screening tests performed on
> potential donors. Results Of 1091 potential donors, 3.3% were positive on
> screening serology, including 6 syphilis, 17 hepatitis B, 3 hepatitis C, 6
> HTLV and 4 HIV.

I'd expect the prevalence of actual undiagnosed HIV infection in the
population of lactating women applying to be milk donors to be quite a
lot lower than this (which is around baseline prevalence for the entire
female population) - even based purely on demographics and before taking
into account other factors.

How many of these were false positives? What sort of testing was used,
specifically? Syphilis screening in particular throws many false
positives, but the other screening tests all have a false positive rate
also. Even with a highly specific test, the positive predictive value
can become quite low in a population with very low actual prevalence. 

For example, if you use a screening test with 99% specificity and 99%
sensitivity in a population with a prevalence of actual disease of 0.2%,
the chance that a positive screening result is a true positive (PPV) is
only 16%. You need a specificity of 99.8% just to break even; to get
above 90% PPV you need a specificity of 99.98%!
(Hopefully someone will pull me up on my maths if I've made an error; I
used the calculator here for the details.
http://faculty.vassar.edu/lowry/clin2.html )

Thorough confirmatory testing must always be done in the event of a
'positive' report, and without the results of those confirmatory tests,
these data are meaningless.

Either way, however, "informal milk sharing" does not mean "no testing
has been done". There's nothing stopping mothers involved in this
practice getting their own testing. Nor does the presence of some of
these infections mean that milk is automatically dangerous, or that it
is more dangerous than the alternative.


Lara Hopkins

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