Post #2 put the caveat in the right place and I want to underline it. The quoted dead-volume figures for common configurations vary between manufacturers and the ordering is stable: fixed needle least, luer with hub more, luer with filter needle most.
Posting this under the heading it deserves: Fixed versus detachable needles: the trade-offs Everything below is what sits behind that.
I was wrong about fixed versus detachable needles in a thread last spring and I would like to correct it publicly rather than quietly.
The error was in the units, which changed the conclusion by an order of magnitude. Setting out the corrected version, and the way I now check for that class of mistake.
Where I part company with the opening post, and it is a narrow parting.
Agreed on fixed versus detachable needles, with one qualification that I think matters. The reasoning holds for the case as described. Change the starting assumption and it does not, and the starting assumption is the part nobody states.
Sharps containers are a legal requirement for disposal in several jurisdictions and a sensible precaution everywhere. A rigid sealed container is the minimum.
What I would want before treating fixed versus detachable needles as settled: the method, the sample, and whether anyone tried to find the opposite result. Two of the three are usually missing.
Reading back through the fixed versus detachable needles threads from last year, the same three questions come up every time and only one of them has ever been answered properly. That seems like a documentation gap rather than a knowledge gap.
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.
I have changed my mind on this once already, so take it as current rather than settled.
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.
If it helps: the failure mode here is usually boring rather than dramatic.
I came in to disagree and I am leaving without a disagreement.
An honest declaration on fixed versus detachable needles: I have a prior here and it is strong enough that you should weight what I say downward. Stating it rather than hiding it.
Sharps disposal: needles and syringes should be disposed in a sharps container (a rigid, puncture-resistant container). Do not put them in regular trash. Many pharmacies accept filled sharps containers.
None of the above is medical advice and I am not qualified to give any.
I had written a reply contradicting post #13 and deleted it. Here is what survived.
The practical version of fixed versus detachable needles is three sentences long. The rigorous version is three pages and reaches the same conclusion with the conditions attached.
Confirming post #13 from a second method, which matters more than confirming it from a second person.
On fixed versus detachable needles the community has more anecdote than the confidence in this thread implies, and I include my own contribution in that.
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.
Adding it because I spent an afternoon working it out and nobody should have to twice.
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.
Everything in post #17 holds. The case it does not cover is the one I have.
The strongest argument against my own position on fixed versus detachable needles, stated as well as I can state it, since nobody else has yet.
Coring the stopper is a real phenomenon with repeated entries at the same point and blunt needles. Varying the entry point slightly and using a fresh needle prevents it.
Happy to be corrected if someone holds better data than mine.
What I would tell a new member reading about fixed versus detachable needles for the first time: the confident posts are not the reliable ones, and the reliable ones are longer.
Fixed versus detachable needles would be much easier to settle if anyone reported the denominator. Almost nobody reports the denominator.
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.
Adding this to the thread rather than to the wiki, because I am not confident enough for the wiki.
Answering the question post #20 raises rather than the one it answers.
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.
That reframing is the whole thing. The facts I already had.
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.
The rule of thumb is fine; the edge cases are where it earns its keep.
Narrowing post #26, because the general version has more than one answer.
Distinguishing three things in the fixed versus detachable needles discussion that keep getting used interchangeably: the observation, the proposed mechanism, and the recommendation that gets attached to both.
This follows post #26 rather than contradicting it.
Having read the whole fixed versus detachable needles thread before replying: the question in the first post has not actually been answered yet, and three of us have answered a nearby one instead.
Worth separating two things that post #28 runs together.
Gauge is the outside diameter and higher numbers are thinner. Thinner needles hurt less and draw more slowly, which is the entire trade.