Subcutaneous technique — pinch or not, angle, and does aspirating matter?

Three things I have seen contradictory advice on. What does the technique guide actually say and why?

Pinch. The point is to lift the subcutaneous layer away from muscle so a short needle stays in fat. Whether you need to depends on needle length against how much subcutaneous tissue you have there. With a short needle in a site with reasonable fat, a pinch is often unnecessary. With a longer needle or a lean site, it is what keeps you out of muscle.

Angle. Follows from the same logic. 90 degrees with a short needle into a pinched fold; a shallower angle if the needle is long relative to the tissue. The angle is a means to landing in the right layer, not a rule in itself.

Aspirating. Drawing back to check for blood before injecting. It has largely fallen out of routine subcutaneous practice — the sites used are chosen partly because there is no significant vessel to hit, and the manoeuvre wobbles the needle and adds tissue trauma. It persists as advice mostly by inheritance from intramuscular technique, where the reasoning was different.

All three are the same underlying question: is the needle where you meant it to be.

Adding rotation, since it belongs with these and is the one people abandon first.

Rotate across sites and within a site. Not "the left side this week" — actually move a couple of centimetres each time and keep track somehow. Twenty years in, I still use a mental grid, because I know exactly what happens if I do not.

Lipohypertrophy is slow, painless, and by the time it is obvious it has been developing for months. Then your absorption is unpredictable and you have no idea when it started being unpredictable.

The reframe that all three are versions of "is the needle in the right layer" is useful. I had them filed as three separate rules.

Which is generally how the guides are written. Where there is a mechanism, the mechanism is given instead of a rule, on the theory that somebody who knows why can work out the case that is not in the guide.