The Schilling test, once the gold-standard method for diagnosing the cause of vitamin B12 deficiency, is no longer used in clinical practice: the radioactive reagent it required has been unavailable since 2003, and modern blood tests have fully replaced it. This guide explains what the test was, why it stopped being used, and what’s used instead today.
This is general information, not medical advice. If you’re being evaluated for B12 deficiency, current blood-based testing (not the Schilling test) is what your doctor will actually use.
What the Schilling test was
Named after Dr. Robert F. Schilling, the test was designed to determine whether the body was absorbing vitamin B12 normally, and if not, to help pinpoint why. B12 absorption in the small intestine (specifically the terminal ileum) depends on the vitamin first combining with intrinsic factor, a protein produced by cells in the stomach; if intrinsic factor is missing or insufficient, a condition called pernicious anemia results, since B12 can’t be properly absorbed without it.
How the test worked
The Schilling test involved two possible stages. In stage one, the patient swallowed a small dose of radioactively labeled vitamin B12, then received an injection of ordinary (unlabeled) B12 an hour later, to saturate the body’s B12-binding tissues so the radioactive dose wouldn’t simply be absorbed and stored. Urine was then collected over 24 hours and measured for radioactivity; a healthy person would excrete a normal percentage of the radioactive B12 in their urine, since properly absorbed B12 that couldn’t be stored would pass through and be excreted. If stage one results were abnormal (too little radioactive B12 excreted, suggesting poor absorption), stage two repeated the process while also giving oral intrinsic factor alongside the radioactive B12; if this corrected the absorption problem, it confirmed pernicious anemia (a lack of intrinsic factor) as the specific cause, rather than a different intestinal malabsorption problem.
Why the test is no longer used
This is the important, current information: the Schilling test is rarely performed today, largely because the radiolabeled vitamin B12 preparation required for the test has not been commercially available since 2003. Even before that, the test had practical limitations, requiring precise 24-hour urine collection, radioactive material handling, and multiple visits for the two-stage version, all of which made it cumbersome compared with newer approaches. A commonly used single-stage simplified version of the test was also found, in comparative studies, to produce ambiguous or misleading results in a substantial proportion of cases, further limiting its reliability even while it was still available.
What’s used to diagnose B12 deficiency today
Modern diagnosis relies on blood testing rather than the Schilling test’s absorption-based approach: measuring serum vitamin B12 levels directly, testing for intrinsic factor antibodies (which, if present, strongly suggest pernicious anemia as an autoimmune cause), and measuring methylmalonic acid (MMA) and homocysteine levels, which rise when B12 is deficient at a cellular level even before standard B12 blood levels drop into a clearly abnormal range, making these tests useful for catching earlier or more subtle deficiency states that older testing approaches sometimes missed.
Why understanding this history still matters
Even though the Schilling test itself is obsolete, its underlying physiological logic, that B12 absorption depends on intrinsic factor and a functioning terminal ileum, remains medically accurate and still informs how doctors think about and investigate the underlying cause of a confirmed B12 deficiency today, just using different, more practical testing tools to get there.
Frequently asked questions
Is the Schilling test still performed today?
No, it’s rarely performed, primarily because the radioactive vitamin B12 preparation it required has been unavailable since 2003. Modern blood tests have fully replaced it.
What replaced the Schilling test?
Blood tests measuring serum B12 levels, intrinsic factor antibodies, and methylmalonic acid (MMA) and homocysteine levels are now used to diagnose and investigate the cause of B12 deficiency.
What was the Schilling test used to diagnose?
It helped determine whether the body was absorbing vitamin B12 normally, and specifically whether pernicious anemia (a lack of intrinsic factor) was the cause of a B12 deficiency versus another intestinal malabsorption problem.
Why did doctors need two stages of the Schilling test?
Stage one checked overall B12 absorption; if abnormal, stage two added intrinsic factor to see if that corrected the absorption problem, which would confirm pernicious anemia specifically as the cause.
If I’m being tested for B12 deficiency today, will I get a Schilling test?
No, current testing relies on blood tests (serum B12, intrinsic factor antibodies, MMA/homocysteine levels) rather than the Schilling test, which is now considered obsolete.