Measurement of intestinal absorption of 57Co vitamin B12 by serum counting

Abstract

SYNOPSISThe results of the measurement of vitamin B12 absorption by counting the radioactivity of 5 ml. serum obtained eight to 10 hours after the ingestion of an oral dose of 0 5 ,tg. vitamin B12 labelled with 0-5 ,uc. 57Co are compared with those obtained with the urinary excretion (Schilling) test. Inadequate urine collection and impaired renal function were responsible for low results in the Schilling test in four of the 12 control subjects, and an incomplete urine collection in four patients with pernicious anaemia could have led to doubt about the validity of the low result. It is necessary to measure the absorption of vitamin B12 from the small intestine in order to diagnose the cause of vitamin B12 deficiency with precision. The methods available for measuring the absorption of an oral dose of radioactive vitamin B12 include the estimation of radioactivity in the faeces (Heinle, Welch, Scharf, Meacham, and Prusoff, 1952), the urine (Schilling, 1953), the liver (Glass and Boyd, 1957), and the blood (Booth and Mollin, 1956). Before measuring the hepatic uptake of vitamin B12 the residual radioactivity in the bowel must be cleared by laxatives and an enema, and in the faecal excretion method the preparation of the specimens is unpleasant and it takes an average of seven days to complete the test. Because of these disadvantages the measurement of the excretion of radioactivity in the urine (Schilling test) has been the most widely adopted method. However, the Schilling test also has disadvantages: these are that the test depends upon renal function and a complete 24-hour urine collection, and that the large parenteral dose of non-radioactive vitamin B12 may occasionally obscure the diagnosis.The measurement of radioactivity in a singleReceived for publication 9 May 1966. sample of serum or plasma is clearly the simplest method. However, the amount of radioactivity that appears in the blood after oral doses of 0 5 to 1-0 ,ug. vitamine B12 is small and in the early studies in which vitamin B12 labelled with -"Co, 58Co, or 6OCo was used it was necessary either to obtain large quantities of serum or plasma, to count for long periods in very sensitive scintillation counters, or to use isotopes of high specific activity (Booth and Mollin, 1956;Doscherholmen and Hagen, 1957;Goldberg, Trivedi, and Oliver, 1957). In view of these disadvantages the counting of radioactivity in the blood was not regarded as a satisfactory method for measuring vitamin B12 absorption. However, the lower energy gamma radiation of 57Co is counted with high efficiency by scintillation crystals (Eberle and Gleason, 1960), and when vitamin B12 labelled with 57Co became available, blood counting was again investigated and satisfactory results were obtained using a dose of 0.5 ,uc. and a small volume of serum (Nelp, McAfee, and Wagner, 1963; McCurdy, 1965 Studies were made on 12 volunteer control subjects who were hospital patients with either respiratory infections or a myocardial infarction, and on 17 patients with megaloblastic anaemia.S...

Vitamin B₁₂ Absorption Test

“…Normalization of absorption was observed in 8 cases of PA when the test was repeated with IF and in remaining 2 patients the values were subnormal but still dis tinctly higher than the original low values. Recent studies of different workers on 57Co-vitamin B12 absorption-plasma level tests gave similar results but with lower plasma radioactivity values and in some cases equivocal results [1,3,6,9,11,13,14], Probably this is due to use of vitamin B12 with lower specific activity and lower dosage (0.5 //c/0.5 ng to 1.0 fic/l.O //g) and differences in methods of calculation of absorbed vitamin B12. This has generally been done per 1000 ml of plasma [1,11,13,14] or expressed as cpm in 4 or 5 ml of plasma [6,9].…”

“…Recent studies of different workers on 57Co-vitamin B12 absorption-plasma level tests gave similar results but with lower plasma radioactivity values and in some cases equivocal results [1,3,6,9,11,13,14], Probably this is due to use of vitamin B12 with lower specific activity and lower dosage (0.5 //c/0.5 ng to 1.0 fic/l.O //g) and differences in methods of calculation of absorbed vitamin B12. This has generally been done per 1000 ml of plasma [1,11,13,14] or expressed as cpm in 4 or 5 ml of plasma [6,9]. A flushing dose of cold vitamin B12 injection was frequently administered at different time intervals and occasionally carbocal was also given before the test [1,3].…”

Cellular uptake of cobalamin: Transcobalamin and the TCblR/CD320 receptor

“…This test is no longer available. Some success has been achieved by monitoring the appearance of Cbl in blood following a dose of 57 CoB 12 (18, 19). However, radioactive Cbl for this use is no longer available.…”

Plasma Radioactivity Following Simultaneous Oral Administration of Intrinsic Factor-Bound and Free Radioactive Vitamin B₁₂

“…These results, although disappointing, are not unduly surprising since recent studies have demonstrated the practical advantages of using vi tamin B i2, labelled with 57Co rather than with 38Co [8,10]. This situ ation might well be improved by the use of free 57Co-labelled vitamin B,2 and 58Co-labelled vitamin B^ bound to human gastric juice, (a reversal of the materials used in this study).…”

“…It has been shown that there is excellent correlation between 8-hour peak levels of radioactivity in plas ma and 24-hour urinary excretion, in both normal subjects and patients suffering from pernicious anaemia [5,7]. Determination of plasma ra dioactivity has several advantages over measurement of urinary excre tion alone [8,10], A dual isotope ('Dicopac') test for the investigation of suspected vi tamin B12 malabsorption has recently become commercially available. The basis of this test is the simultaneous administration of 2 isotopic forms of radioactive Co-labelled vitamin B12, one of which is free and the other bound to normal human gastric juice.…”