Isnt there a blood test that can check co2 levels in your blood to asses the possibility of sleep apnea?

Not how it works: The very best way to diagnose sleep apnea is by direct observation during an overnight sleep study, and even better an evaluation using c-pap unit. A determination of either pulse oximetry or arterial blood carbon dioxide will be nonspecific, or entirely non-diagnostic.

Sleep apnea: There is a blood test that can check your co2 level. We call the test an arterial blood gas. Requires us to draw blood from an artery, which hurts a bit more than a vein blood draw. This test isn't part of routine testing for sleep apnea, though. We do monitor oxygen and expired co2 as well as heart rate/rhythm and brain waves part of a sleep study. All this monitoring is non-invasive.

CO2: Yes there is blood test available to measure co2.