Perioperative medicine is being developed rapidly as a subspecialty of anesthesiology. Metabolic monitoring, mainly electrolytes, glucose, and ketones monitoring, should be well understood by all perioperative physicians. Electrolytes, or ions, are the charged particles inside the body, either positive ions (cations) or negative ions (anions), which is supposed to be equal. All electrolyte tests can be performed either on plasma or serum, where their concentrations are similar. Sometimes preoperative glucose monitoring is rewarded with first time detection of diabetes. Perioperative dysglycemia is monitored in diabetes, but not defined in nondiabetes. Current recommendations favor moderate glycemic target (140–180 mg/dL) over tight control (80–110 mg/dL). Ketone assay is advocated for both diagnosis and monitoring the underlying disease; however, controversy exists in methods (whether blood or urine assay, laboratory analysis or POC device use, assay of acetoacetate or BOHB or acetone). It is recommended to use laboratory serum analysis for confirmation of capillary blood ketone values.