Does Oral PCN Affect OB GBS Culture Result? - Dr. Chapa’s OBGYN Clinical Pearls

Podcast Summary: Dr. Chapa’s OBGYN Clinical Pearls

Episode: Does Oral PCN Affect OB GBS Culture Result?
Date: October 19, 2025
Host: Dr. Chapa

Overview

This episode explores a real-world clinical dilemma: Does oral penicillin (PCN) therapy at the time of Group B Streptococcus (GBS) culture collection in pregnancy affect the validity of the culture result? Dr. Chapa discusses evidence, guideline gaps, and practical considerations, prompting listeners to think critically about a scenario not specifically addressed in formal recommendations.

Key Discussion Points & Insights

1. Clinical Scenario & The Guideline Gap

Real-World Question: What if a routine OB patient is on penicillin (e.g., for strep throat) when her universal GBS screening swab is collected? Could this temporarily suppress GBS colonization, leading to a false negative?
- “Is that GBS culture result valid? I mean, it came out negative, but the patient was on oral penicillin. Can that actually lower the count temporarily...and we get an automatic kind of a false negative result?” (02:08)
Routine GBS screening: 36–37+6 weeks -- “Universal collection of GBS is between 36 weeks and 37 weeks and 6 days.” (01:32)
Existing guidelines (ACOG Committee Opinion 797, 2020, reaffirmed 2025) do not address this specific situation.

2. Evaluating the Official Guidance

Guidelines consider a negative culture valid for 5 weeks unless otherwise specified.
No mention in guidelines about antibiotic use at the time of swabbing:
- “There's nothing, nothing about a patient on penicillin based antibiotics. All right, so what do you do? This is a real world issue.” (03:38)
Dr. Chapa takes the episode into a "podcast courtroom" to examine both sides:
- Evidence suggesting it shouldn’t matter
- Theoretical and anecdotal evidence that it might

3. Courtroom Arguments: Does It Matter?

A. “It Doesn’t Matter” Side

Following the letter of the guidelines, negative is negative:
- “You've done your diligence there, which is you're looking for colonization. And whether it's natural absence or suppressed absence, it's negative. Negative is negative.” (10:37)
The only reason for repeat screening is if 5 weeks have elapsed and the patient is still pregnant—not related to antibiotic use:
- “The one time where repeating the GBS culture is mentioned...is based on length of time of remaining pregnant after first collection.” (11:06)

B. Counterpoint: “It Might Matter”

There is a single published case report:
- A pregnant woman on oral antibiotics for a UTI had a negative GBS screen. After delivery, her neonate developed GBS sepsis—raising concern that the antibiotics suppressed detection (20:52).
- Reference: Kim et al., "Neonatal group B Strep sepsis after negative screen in a patient taking oral antibiotics" (Obstetrics & Gynecology, 2005).
- “According to this case report, as the authors state, ‘relying solely on a negative culture in the context of recent antibiotic use may not be appropriate for intrapartum management.’” (22:54)
Important nuance:
- IV antibiotics are proven to drop GBS colony counts rapidly and prevent neonatal GBS disease; oral antibiotics have not demonstrated effectiveness to achieve the same (19:50).
- Cited directly from guidelines:
  - “Neither antepartum nor intrapartum oral or intramuscular regimens have been shown to be comparatively effective in reducing GBS early onset disease.” (18:05)

4. Practical Implications & Real-world Decision Making

Guidelines are just that—guidelines:
- “They're not de facto dictums. Okay? They're not mandates. It's just, hey, something to consider because...there's no caveat for that.” (24:07)
Dr. Chapa’s take:
- If a patient is on antibiotics at GBS screening, consider the following pragmatic options:
  - If possible, wait until completion of antibiotic course and re-swab (no established evidence for how long to wait, but a week is reasonable if time allows).
  - If cannot delay, recognize the (very small, but not zero) risk of a false negative.
  - Ultimately, a negative culture remains guideline-valid, but re-screening is a reasonable, conservative option in borderline cases.
- “If the patient is on penicillin therapy and you're getting closer to a gbs, you can consider stopping it before. How much before? I don't know. Because we don't have that guidance...And if you can't do that, then just wait for her to finish and then collect it when she's done.” (25:14)
Judgment call:
- “Sometimes it's not all a guideline and clear science. Sometimes that's where the art of medicine comes in and you have to take something to court and see what the best evidence is.” (26:16)

5. Lab Methodological Note

Culture remains the gold standard for GBS screening, though PCR is acceptable at term for those without a previous culture (27:20).
The reason: need for sensitivity testing and proven clinical validity.

6. Encouragement to Discuss & Question Guidance

Dr. Chapa encourages the audience to bring this type of case to their journal clubs or trainee discussions—acknowledging the value of questioning possible gaps in guidelines:
- “Talk amongst yourselves. Take it to your journal club. Fascinating topic.” (28:04)

Notable Quotes & Memorable Moments

“It's a good one. We don't have a good answer for it because that potentially, potentially is a gap in the guidelines.” (08:45)
“All it takes is one bad case of early onset neonatal sepsis to change the picture on that.” (09:50)
“Guidelines exist and they're just guidelines. They're not de facto dictums.” (24:07)
“Negative culture should be a negative culture. Negative culture should be a negative culture. Until it isn’t.” (28:07)
“Sometimes that's where the art of medicine comes in and you have to take something to court and see what the best evidence is.” (26:16)

Timestamps for Key Segments

| Timestamp | Content/Segment | |-----------|-----------------------------------------------------------------------------------------------------| | 00:00–04:00 | Introduction of the case, outline of the clinical question, and review of standard GBS screening | | 04:01–10:35 | Explanation of existing guidelines and Dr. Chapa’s initial interpretation | | 10:36–12:30 | “Courtroom” presentation—arguments for “it doesn’t matter” | | 12:31–15:56 | Further guideline specifics, criteria for repeating GBS, and validation of negative culture | | 17:44–24:06 | Oral vs. IV therapy, discussion of the Green Journal case report, limitations, and real-world impact| | 24:07–27:00 | Practical management suggestions—balancing art and science, lab/testing mention | | 27:01–End | Summary of implications, encouragement for continued discussion, and Dr. Chapa’s closing thoughts |

Take-home Pearls

Current guidelines do not address oral antibiotic use at the time of GBS screening. Negative cultures are considered valid for five weeks.
Case reports suggest oral antibiotics could potentially cause a false negative, but evidence is extremely limited.
Clinical judgment is necessary—a negative is a negative according to formal standards, but repeat screening post-antibiotics is a reasonable consideration if feasible.
Best practice: Engage in team/peer discussions on guideline “gray zones” and document your rationale when the evidence is unclear.
Culture remains gold standard; PCR is acceptable in some cases.

Final words from Dr. Chapa:
“Negative culture should be a negative culture. Negative culture should be a negative culture. Until it isn’t.” (28:07)

Summary prepared in the spirit and style of Dr. Chapa—clinical, evidence-based, a little bit fun, and always focused on real-world OB practice.

Transcript

A (0:00)

Foreign. Well, in true form, we have an episode today that stems from a real world clinical conundrum, a real world clinical question that one of the other attendings asked in our group chat regarding GBS culture and its result. Now, at first, first take, I know what you're thinking. Wait, what? I mean, group B, strep culture. What about it? I mean, if it's negative, it's negative. If it's positive, then she gets intrapartum prophylaxis to prevent early onset neonatal sepsis. All of that is correct. Totally true. But the question is actually something that kind of came from left field, but is totally applicable because this, once again was in a current patient and it's nowhere in the guidelines. So this potentially, potentially is a gap in the care consensus. So again, I'm not saying we know what to do here once we review this episode, but I am going to give some real world clinical applications and things to consider should you have a patient like this. Okay, because I'm going to give you the question in a minute. But the answer is. Oh, it's pretty darn clear. It's pretty darn evident from the guidelines, comma, and it's not. So those are both of the answers. Well, we know what to do ish. And we actually don't know what to do. So let me explain the question here. So one of the other attendings sent a message in our attending group chat to the faculty channel and said, hey, I've actually not seen this before. And is this valid or not? So here's the situation. Routine OB patient had her routine GBS swab in the correct time. Remember, the current consensus and the universal collection of GBS is between 36 weeks and 37 weeks and 6 days. Or just call it what you want. 38 weeks. It's a two week span where we do universal GBS, vaginal and rectal swab. That goes without saying. Everybody gets that. Unless of course, the patient has a GBS bacteria or a previous child who was affected with early onset neonatal sepsis, for which they are called forever carriers. Okay, so routine screen. Got it. Fine. It was negative. Cool. No problem. Right? That's pretty clear. But here's a catch. The patient was actually on oral penicillin for pharyngitis. Right. For a strep throat. I think it was actually amoxicillin. The point is, it was a penicillin based therapy. So our co attending, one of my partners, put this question out. It's a good question. Hey, is that GBS culture result valid? I Mean it came out negative, but the patient was on oral penicillin. Can that actually lower the count temporarily where we get an automatic kind of a false negative result? I don't know. That's a good question. That is a good question. And the reason it's a good question is because it's not that straightforward. We, we totally, totally have guidance on what to do with this. We definitely do. From ACOG, of course, for universal screening for GBS. That goes back to ACOG's Committee Opinion 797 back in February of 2020, but it was reaffirmed this year earlier, 2025. So we get that it just says screen for GBS 36 to 37 and 6 outside of that criteria that we talked about a little while ago and it says nothing, nothing about a patient on penicillin based antibiotics. All right, so what do you do? This is a real world issue. I mean, it happened to our co attending. It's a good question because penicillin obviously is very effective at dropping bacterial counts of GBS in the vagina. So could it be, could it be that, that could lead to a false negative? Now, right now. So let me give you the yes and the no. The no is there's no guidance anywhere that says that we should not screen for GBS if the patient is on some kind of penicillin. However, if you think, well, that's pretty clear, then I guess we're done. That's the answer. It really doesn't seem to matter. Wrong, sir. Wrong. You get nothing. You lose. Good day, sir. Yep. The truth is it can be possibly a little bit more complicated because there is a case report, although it is a case report, that's an n of 1, where this exact same thing happened and the patient was later found to convert from a negative GBS culture to a positive GBS culture after her antibiotics were taken. What now? So now please don't get upset with me. I'm not suggesting, of course, that we change all of our guidance based on a case report of one and not saying that, but it's something to consider. And this was. We went back and forth in our group chat and so my first stock answer, of course by guideline is, guys, it doesn't matter. There's no specific clause which says we shouldn't collect this at this time because a negative culture is a negative culture. That's valid for up to five weeks. But as the chat started evolving, going, but wait a minute, what if we actually do suppress that and then off suppression, she actually is a carrier and we miss it. So of course I went back and did, you know, a little bit of research and sure enough, I did find that case report, all of which we're going to cover in this episode. Now, there is a caveat. Guys, hold on. The reason we're going to. You want to come back after the intro, which we're still in, is because there's more to that case report than meets the eye. Okay? Because there can be potentially, potentially a difference between responsive GBS bacterial count from IV antibiotic therapy versus oral antibiotic therapy. So once again, if you think, wait, this is easy. This is such a weird topic. It's not a big deal. We screen everybody for GBs regardless of antibiotics, 36 to 37 in six. And so it shouldn't make a difference. That can be correct. However, Wrong, sir. Wrong. You get nothing. You lose. Good day, sir. Yes, that's all my point. It's all my point is something to consider. Make this a part of your OB journal club. Talk with your fellows about this. Is should you have on that weird chance and coincidence that the patient is about to get GBS swab and she's on a penicillin based antibiotic and it's a good question to ask. Is that test going to be valid or are we artificially suppressing the count and then potentially risking not treating a GBS carrier? It's a good one. It's a good one. We don't have a good answer for it because that potentially, potentially is a gap in the guidelines. I found this interesting. So I gathered some data. This is where we're going. Can oral penicillin affect or impact GBS culture results and lead to a false negative? Think about it. We never thought about this before. I'm like, we just collect it. But maybe, maybe just something to consider. We're going to cover both sides of this. Even though we have an n of 1. An n of 1 as a case report doesn't mean it has happened before because people have to write it in and write it up and get it submitted for peer review. So you move the needle. So maybe it's happened more than one, but people don't write it up. So it's something interesting to consider. We're going to do both sides here. As we are. If we are in court, we're going to do the side that says it absolutely doesn't matter. Who cares? It's a negative culture. Her chance of being colonized to a significant level within the next five weeks, which is what the guidance says is extremely low. So once you're negative, you're done. And then the rebuttal, which is an N of 1. One case report, but all it takes is one bad case of early onset neonatal sepsis to change the picture on that. And that child was actually infected and affected by otherwise undetected GBS because of the patient's concurrent penicillin therapy. Fascinating. At the end of this, we're going to give some practical implications based on what we know and what we don't know from this real world clinical situation. It's actually a really good case. I think I've set it up enough. Let's be done with the intro and we'll be right back. This is Dr. Chapa's own OBGYN no Spin podcast. Well, we're taking this question, can oral penicillin based therapy affect a vaginal rectal GBS culture? Result. We're gonna take this to our pseudo podcast courtroom and present both sides of the case as each litigant and defendant give their account of the facts. So here we go. Oh, there it is. This is our court song, I guess. Oh, I like Law and Order. Not a sponsor. Isn't that just a great song? I mean, it's awesome. I mean, really, that. That's got some jam to it. So we're taking this question to our pseudo podcast court to figure out if oral penicillin therapy is guilty of lowering vaginal GBS culture results. Let's. Oh, my goodness. Okay, let's get out of that. Because if not, I can listen to. I can listen to Natural Wild. I think that's got some. Man, that's got some good vibe to it. So right when you first listen to this question, like, ah, is that a big deal? I mean, is this. Is this a thing? Well, we didn't think it was a thing until it happened to us. And I'll tell you what we decided to do in this case. But the truth is, as we get back to the facts and get away from the silliness, there is a true gap here in the guidance. So let's start with the. No, it absolutely doesn't matter. Who cares if she's on oral penicillin, a negative swab. You've done your diligence there, which is you're looking for colonization. And whether it's natural absence or suppressed absence, it's negative. Negative is negative. The one time. The one time where repeating. The GBS culture is mentioned in Committee Opinion 797, which, although it originally came out years ago, it was reaffirmed in 20, 25. There's only one time, and that's based on length of time of remaining pregnant after first collection. Okay? So if you ask on an oral board, what's the one criteria of when you want to repeat a GBS in the same patient in the same pregnancy? And it's very simple. Two things have to be met. Number one, she was negative on the initial screen. Number two, she's still pregnant five weeks later. Because the validity of a positive screen goes down with time. Because after about five weeks, it is possible for the patient to have new acquisition of GBS colonization and to reach a certain level, which is, of course, potentially dangerous for the child. So it's five weeks. All right, so let me read this to you. So the only criteria is initially it was negative at first, and the patient remains pregnant five weeks later. That's it. There's nothing about rescreening. If the first collection was negative and the patient was on antibiotics, there's nothing in there about that. So right now, according to the guidance, it says if the patient remains pregnant five or more weeks after a negative baseline GBS test, then GBS screening should be repeated if a recurrent episode of preterm labor occurs or. Or at 36 to 37 and six weeks of gestation, which is your routine time to screen, end quote. So that's it. That's the only time to rescreen. If they're positive, they're just positive. Or if they've got GBS bacteria, they got GBS bacteruria. And the only time, the only time that rescreening is mentioned in that co. The committee opinion is based on length of time of negativity since its first collection. That's. It has nothing to do, says nothing about whether the patient's on oral antibiotics or not. So. So remember what we're doing here, guys. We're trying to present this as if in a court making both sides. And right now we're presenting the side that says it doesn't matter. A negative culture is in fact a negative culture. You've done what you're supposed to do, move on, you're good for five weeks, case closed. We looked for it, and it's not there. Five weeks. However. However, remember, we're trying to give both sides of the argument here. And there is that case report. I get it, I get it. It's just a case report. It's an n of 1, but potentially there's more, but people don't write it up. And it's just something to consider. I'm just. I'M bringing this up as a potential gap in the guidance when you figure if GBS is so sensitive to penicillin, can taking oral antibiotics temporarily. And that's the catch. Temporarily lower the count where it's not detected by. And then the patient actually can convert quicker? I don't know. We don't. So to be very clear on this side of the court, on the defense side, which is, hey, our test is innocent here. The oral antibiotics here have nothing to do with this. Keep going. That is how we've just presented. When we come back from our break, we're going to cover the next part, which is the case report of N of 1. I get it. But it does open up the eyes that maybe it is potentially something that we should pay attention to. So we'll take a quick break and we come back, we're going to talk about the other side that says that oral antibiotics can be guilty of causing a false negative. Something to consider. We'll be right back. You're listening to the ob GYN no spin podcast. Can I take your order? Can I get a tall chai, a large black coffee?