Insulin syringe graduations: U-100 syringes are marked in units where 100 units = 1 mL. Always confirm your syringe is U-100. Confusion between insulin units and milligrams is the most common syringe error. The reasoning is more useful than the number, which is why I have shown it.
Dead volume across syringe and needle combinations — setting out what I have, and where I think it stops being reliable.
A follow-up question about Dead volume across syringe that I did not know to ask the first time.
The earlier thread answered what I asked. What I should have asked is below, and I think it is the one that matters.
The opening post describes the usual case. This is about the unusual one.
Safety-engineered needles are worth using if you have children or anyone else in the house, and the retraction mechanism occupies dead volume that ordinary syringes do not.
Adding the measurement that post #3 says would settle it.
I have three months of notes on Dead volume across syringe and the honest summary is that the trend is real and the week-to-week numbers are noise. I nearly drew the opposite conclusion from the first fortnight.
Thank you for taking the time. That was more work than a reply usually is.
Answering the question post #3 raises rather than the one it answers.
Offering a way to settle Dead volume across syringe rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
Insulin syringe graduations: U-100 syringes are marked in units where 100 units = 1 mL. Always confirm your syringe is U-100. Confusion between insulin units and milligrams is the most common syringe error.
The reasoning is more useful than the number, which is why I have shown it.
Coming back to post #8, because the follow-up matters more than the original answer.
Fixed versus detachable needles: insulin syringes usually have fixed needles (no dead volume loss to the hub). Luer-lock syringes use detachable needles (more dead volume). Fixed needles are preferred for small doses.
The reason Dead volume across syringe is hard to answer is that the obvious measurement and the relevant quantity are not the same thing, and substituting one for the other is silent.
Answering the question post #10 raises rather than the one it answers.
U-100 means the barrel is graduated for a solution containing one hundred units of insulin per millilitre. For anything that is not insulin, the units are simply hundredths of a millilitre.
Post #12 is the version of this I will quote in future. One addition.
Dead volume: residual liquid in the hub and needle after withdrawal. Typical residual is 0.01 to 0.02 mL for an insulin syringe with a fixed needle. Accounting for it matters if precision matters.
Check what the graduations actually say rather than what you expect. A syringe marked in units and one marked in millilitres look similar at a glance and are not the same instrument.
Happy to be corrected if someone holds better data than mine.
Drawing through a wider needle and injecting through a thinner one is a reasonable approach for a viscous preparation and pointless for an aqueous one.
The claim is narrower than it sounds, and deliberately so.
Building on post #16 rather than restating it.
Needle length: typical lengths for subcutaneous injection are 4-6 mm (short), 8 mm (standard), or 10-12 mm (longer). Longer needles are needed for deeper tissue or if you have abdominal adiposity.
Grateful for the specificity. Vague answers to this question are what sent me looking.
I will take the caveat as seriously as the claim, which is the point of putting it there.
Reusing a needle blunts it and there is no benefit to offset that. The second injection with the same needle is the one people describe as painful.
This is the sort of thing the wiki should carry and currently does not.
Worth separating two things that post #20 runs together.
Needle gauge: smaller numbers (30G) are finer and less painful. Larger numbers (25G) are courser. For subcutaneous injection, 27-30G is typical. Finer needles take longer to draw up but hurt less.
That holds under the stated conditions and I have stated them.
This follows post #20 rather than contradicting it.
Fixed-needle insulin syringes have the least dead volume of any common configuration. At small doses that difference is a measurable fraction of what you meant to deliver.
Same conclusion as the reply above, reached differently, which is mildly reassuring.
Filter needles are for drawing from glass ampoules where particulate is a real risk. For a stoppered vial they add dead volume in exchange for very little.
A detachable luer needle plus hub can retain enough volume to matter. Whether it matters to you depends on your dose, and the arithmetic is worth doing once.
Picking up post #23: that is the part I would want checked first.
One more thing on Dead volume across syringe that took me far too long to see: the two figures people quote are not measuring the same quantity. Once you notice that, the apparent contradiction disappears.
This settles it for me, at least until somebody posts a reason it should not.
Needle length for subcutaneous delivery is a question about tissue depth and injection angle together. Short needles at ninety degrees and longer ones at an angle reach comparable depths.
I would keep Dead volume across syringe and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
If your calculated volume is under two graduations, choose a lower concentration rather than trying to read between marks. Precision comes from the preparation, not the eye.