Window period blood donations

[Guest guest]

Dear Rafique,

Calculating the number of HIV positives in the window period is not easy and can

only be guesswork. The number really depends on the length of the window period

and the number of new infections since, only recently infected cases have the

window period. It does not depend on the HIV prevalence. In nascent epidemics,

it is possible to have a low HIV prevalence and high infection rate.

The tests used at present in blood banks are all either second generation

or third generation tests which use either recombinant or synthetic antigens.

These tests are far more sensitive than the earlier tests and so the window

period is much shorter now and generally less than 25 days in more than 95% of

cases as has been shown in surveys of recently infected regular blood donors who

were tested with , p24 and PCR at the same time.

As an example, let us try to calculate the number of window period donations.

This is only for the purpose of illustration and assumes that the epidemic is

steady state and that the incidence rate is proportional to the prevalence which

does not happen in real life.

In India, the HIV prevalences in many states were more or less static in the

last 3 years. This would mean that deaths were matched by new infections during

this period, provided that the population was steady and the net migration zero.

If we assume that in a particular state, the HIV prevalence is 1%, the length of

life span after infection 10 years, the length of the window period 1month, and

the number of blood donations in a particular year 240,000, we can calculate the

number of probable window period donations in the state. A donor would be in the

window period for only 1 month out of the 120 months of his life when he is

infectious.

From this we can see that out of 240,000 donors, only 1% that is 2400 are likely

to be HIV infected. Of these only 20 i.e. (2400/120) will be window period

donations. Some of these window period donors would test positive in one of the

other tests such as that for syphilis, hepatitis B and hepatitis C which are

also done in every unit of blood donated. But some window period donors will

still be missed. The only way to reduce this risk is to have a good donor

selection program. If only regular voluntary donors are allowed to donate and a

voluntary self exclusion program of those with risk behaviour is instituted in

blood banks, this risk can be greatly reduced.

The rate of new infections and the risk of window period donations are high in

those with high risk behaviour. Hence any one with a high risk behaviour life

style should not be allowed to donate. Now, even people belonging to high risk

groups such as prisoners are not allowed to donate since it is impossible with

current testing strategies to eliminate window period donations. The following

link gives an excellent discussion of the issues involved.

www.webcom.com/mjljweb/jrnlclb/vol1/a10.html

Dr.M. Prasanna Kumar

E-mail: mpkumar@...

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