There is no fixed number of questions you can miss on the SPI exam and still pass, because ARDMS uses scaled scoring rather than a simple percentage cutoff. The exam contains approximately 110 multiple-choice questions, and you need a scaled score of at least 555 to pass. How many raw questions that translates to depends on the difficulty of the specific version of the test you receive.
How the SPI Exam Is Structured
The Sonography Principles and Instrumentation (SPI) exam is administered by ARDMS and is required for candidates pursuing the RDMS, RDCS, RVT, and RMSKS credentials. You get two hours to complete the exam, which includes a brief three-minute survey. The test consists of approximately 110 multiple-choice questions covering ultrasound physics, instrumentation, and related topics.
Some of those 110 questions are unscored “pretest” items that ARDMS uses to evaluate new questions for future exams. These don’t count toward your score, but you won’t know which ones they are while taking the test. That means you need to treat every question as if it counts.
How Scaled Scoring Works
ARDMS does not grade the SPI on a straight percentage basis. Instead, your raw score (the number of correct answers) gets converted to a scaled score that accounts for how difficult your particular version of the exam was. The scaled score range uses 555 as the minimum passing mark on every version of the test.
If you get an easier set of questions, you need to answer more of them correctly to hit 555. If your version is harder, you can get that same scaled score while answering slightly fewer questions correctly. This system keeps the passing standard consistent across test dates, even though different candidates see different questions.
Because of this adjustment, there’s no single “magic number” of questions you can miss. On one form of the exam, missing 25 questions might put you just above passing. On another form, missing the same number could land you just below. The difficulty calibration makes it impossible to pin down an exact count.
A Rough Estimate to Guide Your Studying
While ARDMS doesn’t publish the exact raw-to-scaled conversion, most candidates and prep course providers estimate that you need to answer roughly 70% to 75% of scored questions correctly to pass. On a test with around 110 total questions (some of which are unscored), that general range suggests you can probably miss somewhere around 25 to 35 questions and still reach a 555 scaled score, depending on the difficulty of your exam form.
That estimate is loose by design. Aiming to “just barely pass” is a risky strategy when you can’t predict how many of your missed questions were scored versus unscored, or how the difficulty adjustment will affect your particular test. A safer target is to aim for 80% or higher accuracy in your practice exams before sitting for the real thing.
What the Exam Covers
The SPI focuses on ultrasound physics and instrumentation rather than clinical anatomy or pathology. The major content areas include acoustic propagation and wave behavior, transducer design and operation, pulse-echo imaging principles, Doppler principles, image quality and artifacts, and safety and bioeffects. Understanding how to apply physics concepts to real scanning scenarios is more important than memorizing formulas in isolation.
Many test-takers find that artifacts, Doppler physics, and instrumentation carry significant weight on the exam. Spending extra study time on these areas tends to have the highest payoff, especially if your program emphasized clinical scanning more than the underlying physics.
What Happens If You Don’t Pass
If your scaled score falls below 555, you can retake the SPI exam. ARDMS requires a 60-day waiting period between attempts. You’ll need to pay the exam fee again for each retake. Your score report will show your overall scaled score and how you performed relative to the passing standard, which can help you identify weak areas before your next attempt.