Dextrose content of a (chemical) solution, including blood. Also a pun on dexterity, due to non-intrusive blood sugar monitoring devices which are essentially looking for the glucose content of blood (dextrose is also known as d-glucose) by shining light through one's finger.
A: "I have been feeling very feint lately."
B: "Maybe you have low blood sugar, go get a blood test."
A: "But I am afraid of needles."
B: "Then get a dextrosity check, you pussy."