RBC metabolizes glucose mainly via glycolysis, forming lactate which is released from the cells, this yields 2 ATP for each glucose molecule, much less than complete oxidation, but enough to support the RBC energy needs. There is some oxidation of glucose to CO2 in RBC though.

In humans, RBC cells lack mitochondria and therefore has no functional TCA cycle. The process occurs here is pentose phosphate pathway or shunt.

In this process,  1 carbon of glucose is released as CO2 and energy extracted is used to reduce NADP to NADPH , which function as antioxidant. The resulting 5 carbon sugars are rearranged to a  3 carbon sugar which enters glycolysis again. Hence named shunt  5/6 carbon actually comes back to glycolysis again.

By varying flux through PPP, cells can balance the use of glucose for ATP or NADPH.