[Re: https://www.hifa.org/dgroups-rss/spotlight-group-b-strep-49-question-mar... ]
Thank you for asking for clarification, Neil. Apologies for the confusing wording. To clarify, copying what the CDC Nov. 19, 2010 MMWR “Prevention of Perinatal Group B Streptococcal Disease, Revised Guidelines from CDC, 2010” states under Specimen Collection and Processing for GBS Screening | Timing of Screening”:
"Because GBS colonization status can change over the course of a pregnancy, the timing of specimen collection for determination of colonization status is important. Because colonization can be transient, colonization early in pregnancy is not predictive of early-onset GBS disease (44). Late third trimester colonization status has been used as a proxy for intrapartum colonization (140). The negative predictive value of GBS cultures performed ≤5 weeks before delivery is 95%–98%; however, the clinical utility decreases when a prenatal culture is performed more than 5 weeks before delivery because the negative predictive value declines (37).”
The US obstetrical guidelines for GBS have since been updated at acog.org/gbs <http://acog.org/gbs> where they require permission to post excerpts, but their similar wording is under Timing and Procedure for Preterm Culture-Based Screening.
I defer to others in this forum to expound on the meaning of “negative predictive value” but, to my understanding in regards to your proposed message to the mother, yes, the screening test as you note only tells with certainly (assuming no false negatives, etc.) what’s happening when it was performed, but is still predictive according to studies that have been done.
I suggest this version as a message (similar to what we have in our brochure), "GBS is a normal bacterium that can come and go, so a woman who tests negative might become positive later on. If you test negative, ask your provider about being tested again if you have not given birth within five weeks. Regardless of your GBS status, make sure you and everyone who takes care of your baby know the signs of GBS infection in babies and how to respond."
In regards to "Do we have an estimate for X?” perhaps others here will know of studies with those estimates they could post.
Best,
Marti
CHIFA profile: Marti Perhach is CEO/Cofounder of Group B Strep International, United States. Professional interests: Group B strep disease research and prevention, prenatal infection, One Health. www.groupbstrepinternational.org Email address: marti.perhach AT gbs-intl.org