Phosphate binders and dialysis can have harmful side-effects during the treatments of hyperphosphatemia. Therefore, we evaluated the capability of intestinal bacteria (lactic acid bacteria and bifidobacteria) as phosphate-accumulating organisms (PAOs) for phosphate accumulation, with the aim of determining whether PAO-formulated food can prevent hyperphosphatemia in the early stages. However, methods for estimating microbial phosphate-accumulation capacities require significant improvements regarding specificity, cost, and simplicity. The presented method analyzed cell-free broth to assess the phosphate accumulation capability of cells. Active cells and the constructed phosphate-deficient cells were incubated in assay salt media. After incubation, phosphate-deficient cell-free broth was taken as sample and the blank was the active cell-free broth. Therefore, effects of interfering agents and other metabolites were avoided and enhanced the specificity remarkably. Phosphate contents were assessed by reactions with toluidine blue O. In contrast to the case in previous studies, the shift in the first absorbance peak was found to be inversely proportional to the phosphate concentration. The minimum detectable phosphate concentrations for the 11th isolate of Lactobacillus casei JCM 1134 and 8th isolate of Bifidobacterium adolescentis JCM 1275 were determined to be 1.24 and 0.4 mg/L, respectively. Further, the validation results were found to be significant (p-value < 0.05).