Group B strep culture results typically take 36 to 72 hours, with most labs reporting negative results within 48 hours. The wait depends on which type of test your provider orders and whether the sample needs extra incubation time to confirm a result.
Standard Culture Results: 2 to 3 Days
The standard GBS screening test is a culture, meaning the lab places your sample in a special broth and waits to see if bacteria grow. This process has two stages: first, the sample sits in an enrichment broth for 18 to 24 hours, then it’s transferred to a selective plate for another 18 to 24 hours. That adds up to 36 to 72 hours of total incubation time before a result can be confirmed.
In practice, negative results (no GBS detected) are often reported within about 48 hours, because if nothing has grown by that point, the lab can feel confident calling it negative. Positive results sometimes come back a bit sooner, since visible bacterial growth can appear during the first incubation window. Your provider’s office may take an additional day to review and relay results to you, so expect to hear back roughly 2 to 4 days after your swab.
What the Swab Involves
The test itself is quick and painless. Your provider (or sometimes you, if asked to collect it yourself) swabs the lower vagina and then the rectum using the same swab or two separate ones. The swab goes through the anal sphincter briefly to pick up any bacteria present there. Cervical or external-only samples aren’t accurate enough, and a speculum isn’t used. The swab is placed in a transport tube and sent to the lab, where it needs to arrive within 48 hours of collection.
Rapid PCR Tests: Under 2 Hours
When results are needed faster, some hospitals use a molecular test (PCR) that detects GBS genetic material directly from the swab without waiting for bacteria to grow. These tests return results in 1 to 2 hours, and the most current CDC guidelines support their use, noting they take less than 30 minutes in some versions and have over 90% accuracy.
Rapid tests are most useful in situations where a standard culture wasn’t done in time, such as preterm labor or when someone arrives at the hospital without a GBS result on file. They aren’t typically used for routine screening at a prenatal visit because the standard culture remains more reliable as a gold standard. But if you go into labor before your results are back, your hospital may have this option available.
When Screening Happens During Pregnancy
ACOG recommends universal GBS screening between 36 weeks and 37 weeks, 6 days of pregnancy. This timing is intentional: results stay valid for the window when most people go into labor, and it gives the lab enough time to process the culture well before your due date. If you’re screened at 36 weeks and results take 3 days, you’ll have your answer by early in your 37th week, leaving several weeks of cushion.
Some people worry about going into labor before results come back. If you deliver before your GBS status is known, your care team will make treatment decisions based on other risk factors, such as whether you’re running a fever, your water has been broken for an extended time, or you’re delivering before 37 weeks. In those situations, a rapid PCR test at the hospital can sometimes fill the gap.
How Accurate Is the Culture?
The enrichment broth culture method is considered the gold standard for GBS screening, but it isn’t perfect. Studies comparing cultures to molecular methods have found a false-negative rate of roughly 3 to 4%, meaning a small number of people who carry GBS will get a negative result. This is one reason providers also consider risk factors during labor rather than relying solely on a single test.
A positive result, on the other hand, is highly reliable. If GBS grows in the culture, it’s there. The main limitation is that colonization can fluctuate, so a result from several weeks before delivery reflects your status at the time of the swab, not necessarily the exact day you give birth. That’s why the screening window was moved later in pregnancy, to keep the result as current as possible.
If Your Newborn Is Tested
If there’s concern about GBS infection after birth, a blood culture may be drawn from your baby. These cultures work on the same grow-and-wait principle, but GBS tends to grow relatively quickly in newborns. The average time for a blood culture to turn positive is about 15 hours, and 91% of positive cultures show growth within 24 hours. By 36 hours, that figure rises to 96%, and by 48 hours it reaches 99%. Most hospitals will observe your baby for at least 24 to 48 hours if there’s a reason to suspect infection, waiting for culture results before making further decisions.