Diagnostic value of the serum folate assay
The diagnostic value of the serum folate assay has been assessed in 90 patients, each of whom had a macrocytic anaemia and a low serum vitamin B12 level. Twenty-nine (32%) patients were found to have anaemia due primarily to folate deficiency. The cause of the low serum vitamin B12 levels is uncertain in the 22 (25%) patients with normal or borderline vitamin B12 absorption. The effect of folic acid therapy was studied in four of these patients, and in each case the serum vitamin B12 rose slowly to a normal level.
The serum folate was low in only four (7·5%) of the 54 patients with pernicious anaemia, and the levels rose to normal on treatment with vitamin B12 alone. A high serum folate occurred in eight (15%) pernicious anaemia patients. A normal serum folate indicated the diagnosis of pernicious anaemia or megaloblastic anaemia following partial gastrectomy. However, a normal serum folate and a very low vitamin B12 level was found in two patients with idiopathic steatorrhoea.
It is concluded that the serum folate assay is a valuable routine test in patients who have a macrocytic anaemia and low serum vitamin B12. A low folate level makes the diagnosis of pernicious anaemia unlikely and is a strong indication for full investigation of small intestinal function.