Doctors sometimes rely on blood tests to diagnose vitamin B12 deficiency, but this case shows how tests can give misleading results. A 64-year-old woman had all the signs of severe B12 deficiency—including abnormal blood cells and anemia—yet her blood test showed extremely high B12 levels. The twist? Her immune system was producing antibodies that interfered with the test itself, hiding her true deficiency. Once doctors realized what was happening and gave her B12 injections, she recovered quickly. This case teaches doctors to look beyond numbers and consider when test results don’t match a patient’s actual symptoms.

The Quick Take

  • What they studied: A rare situation where a patient’s blood test showed high vitamin B12 levels, but she actually had severe B12 deficiency that was being hidden by her body’s immune response.
  • Who participated: One 64-year-old woman who came to the hospital with severe anemia and abnormal blood cells.
  • Key finding: The patient’s immune system was making antibodies that interfered with the B12 blood test, causing it to show falsely high results (over 2000 pmol/L) when she actually had dangerously low B12. When doctors gave her B12 injections anyway, her blood counts improved rapidly, proving she truly had B12 deficiency.
  • What it means for you: If you have symptoms of B12 deficiency but your blood test shows normal or high B12, ask your doctor to investigate further. Sometimes test results can be misleading, especially if your immune system is involved. This is particularly important if you have other signs like fatigue, weakness, or abnormal blood cells.

The Research Details

This is a case report, which means doctors documented one patient’s medical story in detail. The 64-year-old woman came to the hospital with severe anemia (low red blood cells) and unusual blood cell shapes. Her blood tests showed very high B12 levels, which seemed to rule out B12 deficiency. However, her symptoms and blood cell appearance suggested otherwise. Doctors discovered she had high levels of anti-intrinsic factor antibodies—immune proteins that attack a substance the body needs to absorb B12. These antibodies were interfering with the blood test itself, making it give false results. When doctors treated her with B12 injections despite the confusing test results, she improved dramatically.

This case is important because it shows a real limitation of standard blood tests. Doctors usually trust B12 blood tests to diagnose deficiency, but this case proves that sometimes the test itself can be fooled by the patient’s own immune system. Understanding this problem helps doctors recognize when they need to look beyond the numbers and consider other evidence, like symptoms and blood cell appearance.

This is a single case report, which is the lowest level of scientific evidence. It describes what happened to one person, not a large group. However, case reports are valuable for teaching doctors about unusual situations they might encounter. The case is well-documented with clear evidence of the immune interference. Readers should understand this describes a rare situation, not something that happens to most people with B12 deficiency.

What the Results Show

The patient presented with megaloblastic anemia, which means her red blood cells were abnormally large and immature. Her blood tests showed dangerously low counts of all blood cell types (red cells, white cells, and platelets). Most confusing was her B12 level: it measured above 2000 pmol/L, which is extremely high and would normally rule out B12 deficiency. However, testing revealed she had very high levels of anti-intrinsic factor antibodies. These antibodies attack intrinsic factor, a protein essential for B12 absorption in the intestines. The antibodies were also interfering with the laboratory test itself, causing false high readings. When doctors gave her B12 injections (bypassing the absorption problem), her blood counts normalized quickly, confirming she truly had severe B12 deficiency all along.

The case demonstrates that pernicious anemia—an autoimmune condition where the body attacks cells needed for B12 absorption—can present with confusing laboratory results. The patient’s rapid recovery with B12 therapy confirmed the diagnosis. The presence of anti-intrinsic factor antibodies is the hallmark of pernicious anemia and explains both her B12 deficiency and the test interference.

This case adds to existing medical knowledge about pernicious anemia and test limitations. While doctors have known that anti-intrinsic factor antibodies cause B12 deficiency, this case highlights an additional problem: these same antibodies can interfere with the blood test used to measure B12. This is a known but underappreciated pitfall in clinical practice. The case reinforces that doctors should maintain suspicion for B12 deficiency even when test results seem contradictory.

This is a single case report describing one patient, so the findings cannot be generalized to all patients. We don’t know how common this test interference problem is. The case doesn’t include information about the patient’s long-term follow-up or whether she remained well on continued B12 therapy. Case reports are useful for raising awareness but cannot prove how often a problem occurs or establish firm treatment guidelines.

The Bottom Line

If you have symptoms of B12 deficiency (fatigue, weakness, numbness, memory problems) but your B12 blood test is normal or high, discuss with your doctor whether additional testing is needed. This is especially important if you have other signs like abnormal blood cells or if you have an autoimmune condition. Doctors should consider testing for anti-intrinsic factor antibodies if results seem inconsistent with symptoms. Confidence level: This recommendation is based on one case, so it should prompt further investigation rather than definitive treatment decisions.

This case is most relevant to people with pernicious anemia or suspected autoimmune B12 deficiency. It’s also important for doctors to recognize this possibility. People with other autoimmune conditions may have slightly higher risk. However, this is a rare situation—most people with B12 deficiency have straightforward test results. You should not assume your B12 test is wrong without medical evaluation.

In this case, the patient showed improvement within days to weeks of starting B12 injections. However, recovery timelines vary by person and how severe the deficiency was. Full recovery of blood counts typically takes weeks to months. Long-term management requires ongoing B12 injections, usually monthly.

Want to Apply This Research?

  • If you have pernicious anemia or B12 deficiency, track your energy levels and symptoms daily (fatigue 1-10 scale, numbness, brain fog) before and after B12 injections to monitor treatment effectiveness.
  • Set reminders for B12 injection appointments (usually monthly) and log your symptoms before each dose. This helps you and your doctor see if your treatment schedule is working or needs adjustment.
  • Create a monthly symptom check-in where you rate fatigue, weakness, tingling, and mental clarity. Share this data with your doctor at appointments to track long-term treatment success and catch any problems early.

This case report describes a rare medical situation in one patient and should not be used for self-diagnosis. If you suspect you have B12 deficiency or pernicious anemia, consult with a healthcare provider for proper evaluation and testing. Do not start or stop B12 treatment without medical supervision. Blood test results should always be interpreted by a qualified healthcare professional in the context of your complete medical picture, symptoms, and medical history. This information is educational and not a substitute for professional medical advice.