In medical settings, “SI” most commonly refers to the sacroiliac joint, the pair of joints connecting your lower spine to your pelvis. But SI is one of those abbreviations that wears many hats in medicine. Depending on the context, it can also stand for suicidal ideation, small intestine, stroke index, or the International System of Units used in lab results. Which meaning applies depends entirely on where you encountered it.
SI as Sacroiliac Joint
This is the most frequent use of “SI” in orthopedic, chiropractic, and general practice notes. The sacroiliac joints link the sacrum (the triangular bone above your tailbone) to the ilium (the large wing-shaped bones of your pelvis). You have one on each side. These joints bear the full weight of your upper body when you stand, walk, or climb stairs, transferring force between your spine and legs.
When a doctor writes “SI joint dysfunction” or “SI joint pain,” they’re describing a problem at this connection point. The pain typically shows up in the lower back or buttocks, sometimes radiating into the upper thigh. It often worsens with prolonged standing, stair climbing, or shifting weight onto one leg. Inflammation of the SI joint is called sacroiliitis, coded as M46.1 in the diagnostic system doctors use for billing and records.
To diagnose SI joint problems, clinicians use a set of five hands-on provocation tests: Gaenslen, thigh thrust, distraction, compression, and sacral thrust. If at least three of the five reproduce your pain, SI joint dysfunction is the likely source. This three-out-of-five rule helps distinguish SI joint pain from other causes of low back pain, since the symptoms overlap considerably with disc problems and hip conditions.
SI as Suicidal Ideation
In mental health and emergency medicine records, “SI” almost always means suicidal ideation. This refers to thoughts about, preoccupation with, or consideration of death and suicide. You might see it in chart notes like “patient denies SI” or “positive for SI,” which simply indicates whether someone reported having these thoughts during an assessment.
Clinicians distinguish between two forms. Passive suicidal ideation involves thoughts about death or not wanting to be alive without any plan or intent to act. Active suicidal ideation involves thinking about specific methods or creating a plan. This distinction matters because it helps determine the level of care and safety planning someone needs.
Several validated screening tools measure SI severity. The Columbia Suicide Severity Rating Scale is one of the most widely used, with a short screener version of just three to six questions. The Patient Health Questionnaire-9, a common depression screening form, includes one item specifically about suicidal thoughts. For people who have already made an attempt, the Beck Suicide Intent Scale assesses the intensity of the desire to die at the time of that attempt.
SI Units in Lab Results
If you’ve seen “SI” next to a lab value, it refers to the International System of Units (from the French “Système International d’Unités”). This is the standardized measurement system used internationally for medical lab results. Countries like Canada, the UK, and Australia report lab values in SI units, while the United States primarily uses conventional units.
The practical difference shows up clearly with blood glucose. In the U.S., blood sugar is reported in mg/dL, while SI units express it as mmol/L. To convert, you divide the U.S. number by 18. So a fasting blood glucose of 100 mg/dL equals about 5.6 mmol/L, and 126 mg/dL (the diabetes threshold) equals 7.0 mmol/L. If you’re reading lab results from another country or using an international health app, this conversion explains why the numbers look so different.
The base SI unit for substance concentration is the mole per cubic meter, though in practice labs use millimoles per liter (mmol/L) for most blood tests. SI units apply to everything from electrolytes and cholesterol to kidney function markers.
SI as Small Intestine
In gastroenterology, surgical notes, and imaging reports, SI can stand for small intestine. You might see references to “SI motility” or “SI function” in the context of digestive disorders, bowel obstructions, or surgical procedures. The small intestine handles most of the body’s digestion and nutrient absorption, and it also absorbs the majority of water from food. When doctors discuss SI transit time or SI imaging, they’re talking about how material moves through or how this section of bowel appears on a scan.
SI as Stroke Index
In intensive care and cardiology settings, SI can refer to stroke index, a measure of how much blood the heart pumps per beat relative to body size. It’s calculated by dividing stroke volume by body surface area. This meaning is largely confined to hemodynamic monitoring in critical care and is unlikely to appear in routine medical records or office visit notes.
How to Tell Which Meaning Applies
Context is everything with two-letter medical abbreviations. A few quick rules of thumb: if “SI joint” or “SI dysfunction” appears in an orthopedic, chiropractic, or back pain context, it’s the sacroiliac joint. If it’s in a psychiatric evaluation, emergency department triage note, or mental health intake form, it means suicidal ideation. If it appears next to a number on a lab report, it refers to the international measurement system. And if it shows up in a surgical or gastroenterology note about digestion, it’s the small intestine.
When in doubt, look at the surrounding words. Medical charts almost always provide enough context within the same sentence or section to make the meaning clear. If you’re reading your own medical records and still unsure, your provider’s office can clarify which meaning was intended.