Well what would you suggest. Until the Star Trek Tricorder is
developed, getting a blood sample will require some sort of needle. The
gauge and the length are the only variables whether it is a vacutainer
type system or a butterfly. Gauges smaller than 22 tend to rip red
blood cells causing hemolysis that interfers with testing.

A while ago someone invented a wrist glucometer that aggitated the skin
to leak fluid, and the glucose level was tested with that fluid. I'm
not sure of the clinical accuracy of that device, but it might lead to
other type of testing devices.

But for now, testing blood, serum or plasma requires a needle.

If the phlebotomy is done correctly the patient should not experience
tremendous pain. If the vein is extremely difficult to locate there are
several tried and true techniques to use. Warming with a heel warmer or warm
towel will often bring the veins to the surface. An extreme measure would be
to locate the veins by ultrasound but this is usually only done for
inserting IV lines. As a medical technologist and a patient who is difficult
to have blood drawn from I've seen just and experienced most all techniques.
Its when the phlebotomist insists on probing with needle that is by far the
most painful.