The age distribution of the screened population does affect the Initial 
Positive Rate, but if all pregnant women are screened the effect of the 
different age distributions in different areas is relatively small. Indeed 
as the geographical area screened increases it becomes less and less 
important, and as Brian says, the crucial factor is equality of service.

However:

There are two adverse outcomes after a Down's screen:

1- Having a baby with Down's syndrome.
2- Foetal loss of a normal baby as a result of amnio/CVS. (Some patients 
would include loss of a Down's foetus as a result of amnio, though in theory 
these patients shouldn't be having the screen).

The relative importance of these two outcomes is seen differently by each 
individual The use of a 5% amniocentesis rate when screening by age alone 
was presumably(??) designed to balance these risks in some way, or at least 
limit the cost. As the technique of amnio improves and results in less 
foetal loss should the cut-off be changed as the balance of risks changes?

In public health terms, even though the 5% Initial Positive Rate is fairly 
arbitrary it is at least a hard number which can be used in health 
planninmg. However, if the risk cut-off is too severe, we are making value 
judgements that patients should really be allowed to make for themselves 
about how much risk they are prepared to accept of each adverse outcome. A 
reasonable compromise that allows planning is to use a risk cut-off above 
which amnio would be funded. This is not the same as saying all women with a 
risk greater e.g. than 1:250 are offered an amniocentesis. In short there 
needs to be a better evidence base for how much service the pregnant woman 
wants funding.


>From: [log in to unmask]
>Reply-To: [log in to unmask]
>To: [log in to unmask]
>Subject: Down's screening
>Date: Tue, 25 Jul 2000 09:58:45 +0100
>
>Craig is correct in saying that the false positive rate is dependent on the
>population being screened.
>However, this is looking at the problem from the wrong end. The 5% FP is an
>arbitrary figure and relates to the amniocentesis rate before biochemical
>screening was around. It is now used as a guideline to ensure firstly that
>the workload for cytogenetics labs is not increased too much and secondly 
>to
>avoid subjecting too many women to the risks of amniocentesis.
>The crucial factor if the same service is to be available to everyone,
>wherever they live, is that they are all offered a diagnostic test when 
>they
>reach the same risk - ie the cut-off should be the same. This cut-off will
>need to be judged on the basis of the population-wide (ie National) FP rate
>that this will generate and whether this is consistent with the aims in my
>second paragraph.


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