OB/GYN vs SPI: ARDMS Exam Comparison
This page compares the ARDMS Obstetrics & Gynecology (OB/GYN) and Sonography Principles & Instrumentation (SPI) exams side by side. They differ in clinical scope, content focus, career setting, and 2025 pass-rate data.
OB/GYN vs SPI: Exam Comparison
| Factor | Obstetrics & Gynecology (OB/GYN) | Sonography Principles & Instrumentation (SPI) |
|---|---|---|
| Credential earned | RDMS | RDMS/RVT prerequisite |
| Question count | 170 | 110 |
| Exam duration | 3 hours | 2 hours |
| Passing score | 555/700 | 555/700 |
| 2025 first-time pass rate | 80% | 74% |
| 2025 repeat pass rate | 50% | 47% |
| 2025 overall pass rate | 75% | 68% |
| Difficulty level | Easier | Hard |
Source: ARDMS/APCA Global Exam Performance Summary Report, 2025. First-time and repeat rates describe different candidate groups and do not by themselves establish exam difficulty.
About the Obstetrics & Gynecology (OB/GYN) Exam
The ARDMS OB/GYN specialty exam has the highest first-time pass rate among all seven specialties (80% in 2025), but its content breadth is substantial. It covers the full spectrum of obstetric and gynecologic ultrasound: normal and abnormal pregnancy, fetal anatomy survey, biometry, Doppler surveillance, and gynecologic pathology.
About the Sonography Principles & Instrumentation (SPI) Exam
The ARDMS Sonography Principles and Instrumentation (SPI) exam is the physics prerequisite required before earning any ARDMS specialty credential. It tests sound wave physics, transducer construction, image formation, ultrasound artifacts, Doppler principles, quality assurance, bioeffects, and ALARA safety principles. Unlike specialty exams, the SPI rewards an academic rather than clinical approach.
Key Topics Compared
OB/GYN Key Topics
- First trimester assessment
- Fetal biometry
- Placental evaluation
- Amniotic fluid assessment
- Ectopic pregnancy
- Uterine pathology
- Ovarian pathology
- Cervical assessment
- Multiple gestation
- Fetal anatomy survey
- Growth restriction
- Gynecologic imaging
SPI Key Topics
- Sound wave physics
- Transducer technology
- Image formation
- Spatial resolution
- Artifacts
- Doppler principles
- Hemodynamics
- Bioeffects and safety
- ALARA principle
- Image quality optimization
- Digital signal processing
- Harmonic imaging
Among 2025 candidates, OB/GYN first-time and repeat pass rates were 80% and 50% (a 30-point gap); SPI rates were 74% and 47% (a 27-point gap). These are separate groups, not a measure of individual improvement.
Which Exam Should You Take First?
The right choice depends on your clinical background, your current employer's needs, and your long-term career goals. Sonographers with strong clinical experience in obstetrics & gynecology imaging should consider the OB/GYN exam first, as applied clinical knowledge transfers directly to board performance. Similarly, those working predominantly in sonography principles & instrumentation settings will find the SPI exam more immediately supported by their daily practice.
- Career goals: The SPI is the physics exam every ARDMS credential shares; the OB/GYN exam decides which specialty your RDMS covers. Consider whether that specialty aligns with your desired work environment.
- Where to start: SPI is required for every RDMS and RVT credential, and most candidates take it first. Pass rates compare separate candidate groups, not the inherent difficulty of an exam.
- Clinical experience: Take the exam that aligns with your strongest clinical rotations or work experience. Hands-on scanning experience impacts exam readiness.
- Prerequisite: The SPI physics exam is a prerequisite for earning the RDMS specialty credential. If you have not passed SPI, plan for it before completing the specialty registration.
Both exams reward structured, domain-aligned preparation. The SPI first-time pass rate is 74% compared with 80% for OB/GYN; use these cohort results as context rather than a prediction of your individual outcome.