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.