1.   Most labs test blood samples, but some use cheek cells, a less invasive method that they say is just as accurate.

2.   Researchers are studying a less invasive method.

3.   While the pill has been a gem for Pfizer, it may hurt its competitors, all of whose impotence treatments involve invasive methods.

4.   However, from time to time, blood still needed to be taken using the invasive method to check the oxygen level co-relation.