A cytological screening was an analysis of the cells of the body, sometimes used to determine blood type and donor compatibility. In 2371, The Doctor was required to do a complete pyrocyte replacement to the sole survivor of a Kazon raider in order to keep him alive. In preparation of the procedure, Kes offered to run a cytological screening of USS Voyager's crew in order to find a compatible donor. (VOY: "State of Flux")