Diagnostics Questions for the RNC-OB Exam — Practice Free

ACOG continues to support the Two-Step approach as the standard. Screening is performed between 24-28 weeks gestation. Step 1: 50g Glucose Challenge Test (GCT) non-fasting. If results exceed the threshold (130, 135, or 140 mg/dL), proceed to Step 2. Step 2: 100g 3-hour Oral Glucose Tolerance Test (OGTT) fasting. Diagnosis requires 2+ abnormal values. Note: The One-Step (75g OGTT) is an alternative recommended by the ADA/IADPSG but is associated with higher GDM prevalence. Clinical Tip: Patients with a 50g GCT result ≥200 mg/dL are often diagnosed with GDM without the 3-hour test.

Nucleic Acid Amplification Test (NAAT) using a vaginal, cervical, or first-catch urine swab. Rationale: NAAT is the gold standard due to its high sensitivity and specificity. Vaginal swabs (either clinician- or patient-collected) are the preferred specimen type as they are as sensitive as cervical swabs. Exam Strategy: NCC emphasizes that clean catch is NOT required for NAAT urine samples—in fact, the first-catch (initial 20-30mL) is preferred because it contains the highest concentration of organisms. Avoid choosing culture unless the question specifically mentions antibiotic resistance testing.

Preterm labor is defined as regular uterine contractions AND cervical change (dilation ≥2 cm or effacement ≥80%) OR regular contractions and an initial cervical exam that changes over time. fFN assists when the diagnosis is uncertain (e.g., contractions with <2 cm dilation). A negative fFN effectively rules out PTL for the immediate 7-14 days, whereas a positive fFN in a symptomatic patient necessitates treatment. Exam Tip: Distinguish between threatened PTL (symptoms without change) and true PTL (symptoms with change) when selecting the appropriate use of fFN.

The score is 0. For the Amniotic Fluid Volume (AFV) component of the BPP, the standard requirement for 2 points is at least one vertical pocket of fluid measuring >2 cm (Maximum Vertical Pocket/MVP) that is free of umbilical cord or fetal parts. **Exam Strategy:** Distinguish between BPP (which uses MVP) and the Modified BPP (which often uses AFI). A pocket <2 cm vertical depth is considered oligohydramnios in the context of BPP scoring, indicating potential chronic placental insufficiency.

In FGR, the fetus shunts blood to the brain. This causes Brain Sparing: 1) Umbilical Artery: Increased resistance (High S/D, AEDV, or REDV). 2) MCA: Decreased resistance (increased diastolic flow) as the cerebral vessels dilate. Key Exam Point: A low Cerebroplacental Ratio (CPR <1.0), calculated as MCA-PI / UA-PI, is a highly sensitive marker for adverse neonatal outcomes, even if individual values are borderline.