The proposed GC-FID was applied to evaluate the concentration of methylmalonic acid in human urine samples. The results obtained using the proposed GC-FID methods in the determination of concentration of methylmalonic acid in thirty clinical urine samples were compared with those obtained by the gold standard GC-MS method. As can be seen in Fig. 3, the correlation coefficient between GC-FID and GC-MS was 0.997. This correlation indicates influence the detection precision and sensitivity of the GC-FID, when GC-MS are not available.