ASSAY OF GASTRIC INTRINSIC FACTOR IN THE DIAGNOSIS OF ADDISONIAN PERNICIOUS ANAEMIA

High incidence of type II autoantibodies in pernicious anaemia

AIMS

To investigate the incidence of type II autoantibodies to intrinsic factor in pernicious anaemia.

METHODS

Three hundred and forty four serum samples submitted for intrinsic factor antibody (IFAB) analysis on clinical or laboratory grounds were tested by an established radioassay and a new enzyme linked immunosorbent assay (ELISA) method for type I and total IFAB, respectively. Sixty of these were found to be positive by ELISA; this method was used to test further, 40 samples of adequate volume for types I and II antibodies.

RESULTS

Type II antibodies were detected in 39 of the 40 sera tested. A comparative analysis indicated that seven samples contained pure type II antibody, being positive for total and type II by ELISA, but negative for type I by both the ELISA and radioassay technique.

CONCLUSIONS

The occurrence of type II antibody, both alone and in combination with type I, seems to be more common than has previously been recognised, and emphasises the advantage of using a technique which will detect both types of antibody.

Congenital intrinsic factor deficiency in a Spanish patient

Vitamin B-12 deficiency was diagnosed in a 26-year-old man. Examinations performed to determine the etiology of the deficiency showed a vitamin B-12 malabsorption in the Schilling test which was corrected by adding intrinsic factor (IF) as well as normal gastric mucosa and acid secretion, although IF in gastric juice was absent. Family study showed normal serum vitamin B-12 levels in the parents, who are first cousins, and siblings. A gastric examination in the father and the sister showed decreased IF secretion, indicating heterozygosity for the disorder.

New enzyme immunoassay for detecting total, type I, and type II intrinsic factor antibodies

A method for the detection of total, type I, and type II intrinsic factor antibodies was devised. The technique comprises a two-site solid phase enzyme linked immunosorbent assay (ELISA), with human intrinsic factor conjugated with horseradish peroxidase as label and attached to polystyrene tubes as solid phase. One conjugation provides sufficient material to assay more than 10,000 patient samples. The label proved stable during the course of this evaluation and was still in use more than 12 months after preparation. When applied to 45 serum samples from cases of pernicious anaemia, intrinsic factor antibodies were shown in 30 (67%). Simplicity, high capacity, low cost and label stability, combined with relatively high clinical sensitivity make the method suitable for cost effective screening of large numbers of samples. Simple modifications to the basic assay reagents permitted type I and type II intrinsic factor antibodies to be differentiated.

Intrinsic factor antibody tests

The sensitivity of methods to detect antibodies to intrinsic factor was assessed. Five sera of known antibody content were tested in 31 laboratories and 30 sera from patients with pernicious anaemia were tested in one laboratory. Five non-commercial methods and two kits for type I antibodies and one non-commercial method for types I and II antibodies are in current use. Differences in sensitivity of the non-commercial methods for type I antibodies related more to the antigen: antibody ratio in the test system than to the method itself. A radioimmune assay for types I and II antibodies showed the best sensitivity but that of an enzyme linked immunosorbent assay (ELISA) method was poor.

Laboratory diagnosis of megaloblastic anaemia: current methods assessed by external quality assurance trials

The results of an Interregional quality assurance scheme for tests in the diagnosis of megaloblastic anaemia were reviewed to assess the methods used. Serum folate assays showed great variation between methods, partly due to limitations in assessment by external quality assurance. Red cell folate assays yielded widely different results and much imprecision due both to the differences in preparation of the haemolysate and to the problems inherent in radioassay of a mixture of folate compounds. Intrinsic factor antibody tests showed appreciable variation in sensitivity. There was considerable inconsistency in the detection of polymorph nuclear hypersegmentation.

Vitamin B12 binders (transcobalamins) in serum

The measurement of plasma or serum vitamin B12 (B12) binders is important in the diagnosis of congenital megaloblastic anemias, the myeloproliferative disorders and perhaps in other malignancies. Two classes of binders circulate in plasma, Transcobalamin II (TC II) and R-binders. The latter have been divided by some authors into transcobalamin I (TC I) and transcobalamin III (TC III), and the validity of this division is discussed. R-binders and TC II differ in their apparent molecular weight on gel filtration chromatography, by which method they can be separated reliably. However, the technique is time-consuming and cumbersome and a variety of rapid separation methods have been described in the literature. These are discussed and compared to the standard gel filtration separation method. TC I and III are separable on the basis of their differing charges, and the methods which have been described for accomplishing this are compared and critically reviewed. There has been some controversy in the literature as to whether plasma or serum should be used to measure circulating B12 binders. Leukocytes, which contain R-binders, release these in vitro and this release is greatest when blood is allowed to clot and the serum separated. Consequently, the use of plasma, sometimes with agents added to prevent leukocytic release of binder, has been advocated. Yet, none of the agents used eliminates artifact totally, and this aspect, too, is reviewed. Lastly, several techniques for the purification of B12 binders have been described. Some techniques result in pure binder preparations, while others result in preparations which are free of other binders but contaminated with non-B12 binding proteins. Both approaches have advantages and disadvantages, and these are discussed.

Clinically significant vitamin B12 deficiency secondary to malabsorption of protein-bound vitamin B12

Protein- (chicken serum) bound [57Co]cyanocobalamin absorption was evaluated in five hypochlorhydric patients who had developed B12 deficiency despite having normal absorption of unbound crystalline vitamin B12. All five patients had decreased urinary excretion of protein-bound B12 (0.06--0.34%) as compared to twelve normal controls (0.61--5.6%), P less than .001. Improvement in protein-bound B12 absorption in four of the five patients occurred with the exogenous administration of hydrochloric acid, pepsin, gastric intrinsic factor, or a combination thereof. Vitamin B12 deficiency developing in the setting of hypochlorhydria may result from deficiency of acid-peptic digestion of B12 bound to protein and/or a relative deficiency of intrinsic factor. This digestive defect is not detected with tests which measure the absorption of unbound crystalline B12 but is detected by a simple test which employs B12 bound to chicken serum as the form of protein-bound B12.

Influence of age and stomach function on serum vitamin B12 concentration

Serum vitamin B12 (B12), maximal gastric acid output (MAO), and B12 absorption were determined in 82 subjects, age 32 to 85 years, who had normal absorption of radiocyanocobalamin. In 46 of the patients the gastric intrinsic factor (IF) was also measured. Serum B12 concentration and MAO varied widely in all age groups from abnormally low to definitely normal. The mean values for serum B12 and MAO, however, declined very similarly with advancing age owing to an increased incidence of low values for these two measurerment in the aged. There was no significant fall in the mean B12 absorption or IF secretion as a function of old age. Achlorhydric and hypochlorhydric patients invariably had lower mean serum B12 concentrations than those with adequate MAO. Conversely, patients with normal MAO all had normal serum B12 levels. Serum B12 concentration, although correlating with both MAO and IF secretion, showed a closer relationship to the former than to the latter. These findings suggest that the stomach, aside from producing the IF, plays an important role in maintaining a normal serum B12 concentration.

Vitamin-B12 status of patients on long-term metformin therapy

Vitamin-B(12) malabsorption has been found in 21 (30%) of 71 diabetic patients taking long-term metformin therapy in addition to dietary management. The patients with evidence of B(12) malabsorption had significantly lower haemoglobin levels (and significantly higher serum folic acid levels) than those with normal B(12) absorption. Steatorrhoea was found in only one patient. Stopping metformin therapy resulted in reversion of B(12) absorption to normal in most patients examined. Four patients with B(12) malabsorption were found to have pathologically low serum B(12) levels. The causes and implications of these findings are discussed and it is concluded that all patients on long-term metformin therapy should have annual serum B(12) estimations.

Effect of Berkefeld filtration on the binding activity of human gastric juice

The intrinsic factor content of Berkefeld-filtered human gastric juice has been studied. This appears to vary with the pH at which filtration is carried out and also between individual filters. Significant losses of intrinsic factor may result from filtration and complete loss when filtration is carried out at a low pH. The most suitable pH for filtration appears to be in the range pH 7 to 8.

Gastric secretory function in polycythaemia vera

Maximal acid output (M.A.O.) and intrinsic factor (I.F.) of the stomach were measured in 16 patients with polycythaemia vera, 4 of whom had a duodenal ulcer and 2 pernicious anaemia. It was found that, in general, the M.A.O. was significantly less than normal unless an active duodenal ulcer was present when the M.A.O. would reach the usual levels for duodenal ulcer subjects. The gastric I.F. output showed a significant correlation with the M.A.O. These results are discussed in the light of the increased incidence of peptic ulcer in polycythaemia vera and the occasionally reported cases of polycythaemia vera and pernicious anaemia.