Many thanks for your latest post Marti (#48), this clarifies. I think I misinterpreted your original statement ""... 95% to 98% accurate if done < 5 weeks before delivery, but after that the negative predictive value declines." I interpreted the words 'after that' to mean the period after '5 weeks before delivery' (eg 35-40). Your response clarifies that you meant that the negative predictive value decreases when the test is done earlier ie more than 5 weeks before delivery.
My understanding of “negative predictive value” means how reliable a negative test result is — in other words, if your test says you don’t have GBS today, how likely is that to be true.”. At the time of screening at around 35-37 weeks, the negative predictive value is very high, around 95-98%. However, if a mother has a negative screening test, this may be accurate to tell her if she has colonisation with GBS at that time, but the mother may yet become colonised later in pregnancy.
So perhaps we need two separate terms: one to describe the point accuracy of the test (negative predictive value) and one to describe the risk of colonisation in the final weeks of pregnancy after a (true) negative screening.
The message to the mother might be something like: "GBS is a normal bacterium that can come and go, so a woman who tests negative at 35 weeks might become positive later on. The screening test is at least 95% accurate but it only tells us what’s happening on the day it’s done. It can't predict what happens in later weeks. About X% of women with negative screening result subsequently become colonised by GBS in later weeks." Do we have an estimate for X?
As you say, a rapid test at the start of labour would be very helpful, especially if the results could be obtained within minutes. It may not provide adequate time for IV antibiotics for those 10-20% of labours that last less than 4 hours. But a positive result would be a signal to monitor very carefully the newborn infant.
If I have misunderstood or misrepresented anything in the above, please let us know!
Thanks, Neil
HIFA profile: Neil Pakenham-Walsh is coordinator of HIFA (Healthcare Information For All), a global health community that brings all stakeholders together around the shared goal of universal access to reliable healthcare information. HIFA has 20,000 members in 180 countries, interacting in four languages and representing all parts of the global evidence ecosystem. HIFA is administered by Global Healthcare Information Network, a UK-based nonprofit in official relations with the World Health Organization. Email: neil@hifa.org