I commented to a pregnant friend a few weeks ago that I had never been tested for Group B Strep colonization with either of my two pregnancies, and that I would rather not be tested as I might have to do somaething about it if I found out I were GBS+. She reacted with a bit of shock, and after thinking about it I realized that I actually didn't have a handle on how rare GBS is and whether it was a good idea to avoid testing.
GBS is a bacterium that normally inhabits the digestive tract but occasionally poses a risk of disease, particularly to newborns. Nobody was really talking about it much when I was preparing for my first birth, but these days it seems to be the latest scare. And there a little bit of reason for it: If mothers transmit GBS to their newborns during delivery, 20% of those babies will die, and many of the survivors will have ongoing problems. Scary, as I said. Because of the standard protocol for GBS+ mothers is to administer IV antibiotics, which reduces transmission by 70%.
For mothers who birth in the hospital, this intervention is a small matter. There are already so many interventions going on --- what's a little IV? And a lot of those mothers were going to have an IV anyway. So it's not a big deal, relatively speaking, to add yet another layer of intervention.
But for a mother who was planning to minimize intervention, adding an IV can be a big deal --- they tend to restrict movement. And for a mother who planned a homebirth, adding an IV means changing everything --- because it usually means moving from home to hospital. Is this layer of protection worth it?
To know this, we need to know the risks of transmission in the first place. (The numbers in what follows are from the CDC.) Before GBS prevention strategies were implemented widely, the incidence was 1.8/1000 live births in the U. S. If this still reflects the risk of transmission overall, the chances of delivering a baby free of GBS disease are 99.82% if the mother's culture is unknown and if she does not undergo IV antibiotics.
Women who test positive for GBS are 25 times more likely than women who test negative to have a baby with GBS disease. Let's be conservative (on the side of testing) and assume, for the moment, that virtually all of the infections come from women who would test positive if they were tested. Ten to 30% of women are colonized. If ten, then the chances of GBS disease among babies of women known to be positive would be 1.8 percent; if thirty, 0.6 percent.
IV antibiotics reduce the risk of transmission by 70 percent: from a range of 0.6-1.8%, to a range of 0.18-0.54%.
So let's take a look at the case for testing.
If you don't test and don't do anything:
- Chances are 99.82% that you won't infect a baby with GBS disease.
- Chances are 99.96% that your baby won't die of GBS disease.
If you take the test:
- Chances are 10-30% that you will test positive.
If you are positive and you don't undergo IV antibiotics:
- Chances are 98.2-99.4% that you won't infect an infant.
If you are positive AND you undergo IV antibiotics:
- Chances are 99.46-99.82% that you won't infect a baby with GBS disease.
So let's see. Since I am deciding whether to (a) not test or (b) test and, if positive, use antibiotics, I am deciding whether to take a step that would improve my perception of the outcome from, at best, 99.82% favorable to... at best, 99.82% favorable.
The real outcome is improved more than that, because the real outcome includes the 70-90% chance that I would test negative. Including that, the outcomes with testing and prophylaxis are improved to 99.95 percent.
To sum up:
Going from unknown GBS status to testing and planning to use IVA if positive: improves chances from 99.82% to 99.95% (reduces risk from 0.18% to 0.005%)
Going from positive GBS status, no IVA, to positive GBS status, using IVA: improves chances from 98.2% to a maximum of 99.82% (reduces risk from 1.8% t0 0.18%)
My conclusion is that it's not worth testing. But that's me. What do you think?
(UPDATE. Perspective. Total neonatal mortality in the U. S. from all causes was, in 2001, 4.5/1000 live births --- your chances that your baby will not die in the first 28 days of life are "only" 99.55%.)
UPDATE AGAIN. Only a culture taken in the last five weeks of pregnancy is predictive. Testing before the 35th week is useless!
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