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SCHILLING TEST (24 HOUR URINARY EXCRETION TEST)

 Schilling test is done to detect vitamin B12

deficiency as well as to distinguish and detect lack of IF

and malabsorption syndrome. The results of test also

depend upon good renal function and proper urinary

collection. Radioisotope used for labeling B12 is either 58Co

or 57Co. The test is performed in 3 stages as under:

Stage I: Without IF.  The patient after an overnight fasting

is administered oral dose of 1 μg of radioactively labelled

vitamin B

12 (‘hot’ B12) in 200 ml of water. At the same

time, 1 mg of unlabelled vitamin B12 (‘cold’ B12) is given

by intramuscular route; this ‘cold’ B12 will saturate the

serum as well as the tissue binding sites. The patient is

kept fasting for a further period of 2 hours, following

which urinary excretion of B12 is estimated:

In normal individuals, 24-hour urinary excretion is

>10% of the oral dose of ‘hot’ B

12.

Patients with IF deficiency excrete lower quantity of

‘hot’ B

12 which is further confirmed by repeating the test

as in stage II given below.

Stage II: With IF.  If the 24-hour urinary excretion of ‘hot’

B

12 is low, the test is repeated using the same procedure

as in stage I but in addition high oral dose of IF is

administered along with ‘hot’ B12.

If the 24-hour urinary excretion of ‘hot’ B12 is now

normal, the low value in first stage of the test was due to

IF deficiency (i.e. pernicious anaemia).

Patients with pernicious anaemia have abnormal test

even after treatment with vitamin B

12 due to IF deficiency.

However, abnormal 24-hour urinary excretion of ‘hot’ B12

is further investigated in stage III for a cause in intestinal

malabsorption of ‘hot’ B’12.

Stage III: Test for malabsorption of vitamin B12.  Some

patients absorb vitamin B12 in water as was stipulated in

the original Schilling test. Modified Schilling test employs

the use of protein-bound vitamin B12. In conditions

causing malabsorption, the test is repeated after a course

of treatment with antibiotics or anti-inflammatory drugs.

3. SERUM ENZYME LEVELS. Besides Schilling test,

another way of distinguishing whether megaloblastic

anaemia is due to cobalamine or folate is by serum

determination of methylmalonic acid and homocysteine

by sophisticated enzymatic assays. Both are elevated in

cobalamine deficiency, while in folate deficiency there is

only elevation of homocysteine and not of methylmalonic

acid

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