© 2018, Metrology Society of India. From a standardized and validated method to quantify serum electrolytes (Na, K, Ca and Mg) by flame atomic absorption spectroscopy, combined relative uncertainty (CRSU) and the percentage of combined relative standard uncertainty (% CRSU) were determined. For this, analytical grade standards were used and later from a certified reference material (NIST2670a). Previously, the sources of uncertainty used in the measurement using the Ishikawa diagram (cause-effect) were established, including mass concentration, calibration curves, volume, reactive targets, certified reference material, dilution and repeatability. Quantification of the metals was performed using the atomic absorption spectroscopy technique employing a Perkin Elmer model 3100 equipment. The values obtained for the CRSU on average are below 0.030 and for the % CRSU on average they are below 3.1%. These results indicate that the uncertainty values are acceptable since they are below the recommended values for these metals.