Vitamin B12 (Cyanocobalmin)
Sample
- B12 estimation is done on the serum of the patient.
- A fasting blood sample is needed.
- The specimen is stable for 8 weeks at 8 °C.
- 24 hours of urine samples are collected.
Precautions
- Avoid hemolysis.
- Protect the sample from the light.
- Avoid heparin, fluoride, and ascorbic acid.
- Drugs like chloral hydrate increase the B12 level.
- Drugs that decrease the level are alcohol, anticonvulsants, colchicine, oral contraceptives, and aminoglycosides.
Pathophysiology
- Vit.B12 is a complex molecule and this is also called Cyanocobalamine.
- This is a water-soluble and hematopoietic vitamin.
- This has a central cobalt nucleus surrounded by porphyrins like a ring to which are attached nucleotide and aminopropanol.
- This was also called an extrinsic factor.
- For absorption intrinsic factor (glycoprotein) secreted in saliva, and gastric juice. This is produced by the parietal cells of the gastric mucosa.
- Cyanocobalamin is a stable compound that forms dark red needle-like crystals.
- The physiological form of Cobalamin in the serum is Methylcobalamin.
- The deficiency of the Intrinsic factor leads to pernicious anemia.
- Vitamin B12 deficiency may take place due to parasitic infestation like Diphyllobothrium latum which competes with B12.
Vit.B12 functions
- Food produces Haptocorrin (transcobalamin, or R-protein) from the salivary glands.
- It binds the B12 and protects from the acid of the stomach.
- Dietary source of Vitamin B12:
- The diet source is meat, eggs, milk, and milk products.
- This is not found in the vegetable.
- The diet source is meat, eggs, milk, and milk products.
- The deficiency of Vit.B12 is due to the lake of intrinsic factor (IF) by the stomach and intestinal malabsorption.
- In the absence of intrinsic factor, the patient will have pernicious anemia and Vit.B 12 is not absorbed.
- In case partial gastrectomy Vit.B12 deficiency may occur.
- Intestinal malabsorption of Vit.B12 is caused by gastrectomy or ileal resection.
- Malabsorption may be due to tropical sprue, inflammatory bowel disease, intestinal stasis, and overgrowth of bacteria in the colon.
- This may occur by the intestinal parasitic infestation like Dophyllobothrium latum that competes for the vitamin.
- Signs and symptoms of Vit.B12 deficiency:
- There is megaloblastic anemia (pernicious anemia).
- Patients with pernicious anemia may have atrophic gastritis.
- There is an increased incidence of gastric carcinoma.
- There may be neuropathy.
- Sometimes there is an irreversible neurological loss like burning pain or loss of sensation in the extremities.
- There may be weakness, paralysis, or spasticity.
- The patient may develop dementia, confusion, and disorientation.
- There is megaloblastic anemia (pernicious anemia).
Normal
Source 1
Age | pg/mL |
Newborn | 160 to 1300 |
Adult | 200 to 835 |
60 to 90 years | 110 to 770 |
Other sources
- Newborn = 160 to 1300 pg/mL, (118 to 959 pmol/L)
- Adult = 200 to 835 pg/mL (148 to 616 pmol/L).
- Critical value :
- Decreased B12 level = <100 pg/mL.
- Increased B12 level = >700 pg/mL.
- Some references use:
- Normal value as 200 to 900 pg/mL OR
- Other sources = 206 to 678 ng/L.
- Another reference = 110 to 800 pg/mL.
- Normal value as 200 to 900 pg/mL OR
Diagnosis
- The indirect method measures the urinary and serum levels of methylmalonic acid and homocysteine.
- The Patient deficient in vitamin B12 secrete an excess amount of methylmalonic acid in the urine. Methylmalonic acid can be measured by gas chromatography.
- Homocysteine level is increased in patients deficient in vitamin B12. Because their cells can not metabolize homocysteine to methionine at a normal rate.
- Vitamin B 12 absorption test known as the Schilling test.
- This test can differentiate:
- B12 deficiency (Pernicious anemia)
- Intestinal malabsorption.
- Diet.
- This test can differentiate:
The procedure of the Schilling test:
- This is a two-step test.
- If the first step is normal then no need to do the second step.
- Ask the patient to empty the bladder.
- Step 1: In this test vitamin B12 is radiolabeled with 57′Co or 58′Co-vitamin B12 is given.
- An oral dose of 0.5 to 1.0 µg radiolabelled vitamin B12 is given.
- Within 2 hours the patient is injected with a flushing dose of 1000 µg of unlabelled vitamin B12.
- Radioactivity is measured in the urine, feces, and serum.
- Start collecting 24 hours of urine after both doses.
- Measure the number of radiolabelled B12 in the urine.
- If the amount of radiolabelled B12 measured in the urine exceeds 7.5% of the oral dose.
- Absorption is considered normal and the test is complete.
- The deficiency is considered dietary.
- Step 2: This will differentiate between pernicious anemia and other causes of malabsorption.
- Repeat the doses of vitamin B12 from step 1, but add the intrinsic factor along with the oral dose of labeled B12.
- Collect again 24 hours urine and count the radiolabelled B12.
- If the B12 labeled in urine from step 2 is >7.5% then,
- Malabsorption is corrected by the addition of intrinsic factors and the diagnosis is pernicious anemia.
- If the result is <7.5%, it indicates other causes of malabsorption like Diphyllobothrium latum or celiac sprue.
- Schilling test normal value:
- 15 to 40% of 0.5 µg dose.
- 5 to 40% of 1.0 µg dose.
- 24 hours of urine results are:
- Radiolabeled B12 at the first step:
- >7.5% indicates dietary deficiency.
- The result in step 2:
- >7.5% = indicates pernicious anemia.
- <7.5% = suggest another cause of malabsorption.
- Intrinsic factor (IF) antibodies can be detected in the serum of patients with pernicious anemia.
- 50% of the patient shows these antibodies. While these are absent in other conditions.