Units, millilitres, milligrams: three scales on one small object

The single most useful sentence in this file is that a unit on an insulin syringe is a measure of volume, not of drug. The convention fixes one hundred units to one millilitre, so a unit is ten microlitres and nothing more. How much peptide sits in those ten microlitres depends entirely on the concentration in the vial, which depends on how much powder was in it and how much liquid was added. Every serious dosing error we have documented in this market comes from treating the unit as though it carried an inherent amount of drug. It does not, and it never did.

The hundred-unit convention

An insulin syringe is graduated in units on a convention that fixes one hundred units to one millilitre. A unit is therefore ten microlitres of liquid. That is the entire definition and it contains no information about drug content.

The convention works for insulin because insulin is supplied at a standardised strength: U-100 insulin contains one hundred international units of insulin activity per millilitre, so one syringe unit delivers one insulin unit. The volumetric mark and the dose mark coincide, which is why generations of patients have been able to dose by counting marks without ever thinking about volume.

Take the same syringe and use it for a peptide reconstituted at a concentration of the user’s choosing and the coincidence breaks. The syringe is still an accurate volumetric instrument; it is simply no longer a dosing instrument. Everything a person needs in order to convert marks into micrograms depends on a calculation somebody has to perform.

The Journal labours this because it is the root of the largest errors in this market. A reader who takes away only one sentence should take away this one: the unit tells you the volume, and only the concentration tells you the dose.

Barrel size and graduation interval

U-100 insulin syringes are commonly supplied in three barrel sizes. The 0.3 mL barrel holds thirty units and is usually graduated in single units, with some products marked in half units. The 0.5 mL barrel holds fifty units and is generally marked in single units. The 1 mL barrel holds one hundred units and is very often marked in two-unit increments, because a hundred legible single marks will not fit on a barrel of that length.

The practical consequence is direct. A person accustomed to counting single marks on a 0.5 mL barrel who switches to a 1 mL barrel and counts the same number of marks will draw twice the intended volume. The reverse switch halves it. Nothing about the appearance of the syringe warns of this; only the printed numerals do, and they are small.

The general rule that follows is to choose the smallest barrel that comfortably holds the intended volume, both for graduation resolution and because a small volume measured near the bottom of a large barrel is the least accurate configuration available. Where a dose is genuinely small — a few units — a half-unit-graduated 0.3 mL barrel is the only presentation that offers meaningful resolution.

Reading a barrel accurately

Three reading errors recur. The first is counting marks rather than reading numerals, which fails at the first change of barrel size. The second is reading to the wrong part of the plunger: the measurement is taken at the leading edge of the rubber stopper, not the tip of any conical projection beyond it, and on some designs the difference is a full unit. The third is parallax, which sounds fussy and is not: at small volumes, viewing the barrel from above or below the mark introduces a readable error.

The remedy for all three is the same and takes seconds. Read the numeral, not the count. Hold the barrel at eye level. Identify the leading edge of the stopper before drawing rather than after.

There is a further consideration specific to this market. Syringes sold for general medical use — tuberculin syringes, for instance — are graduated in millilitres and fractions of a millilitre. They are the same size and shape as insulin syringes, and in the same drawer. Using them requires reading a completely different scale, and the failure to notice the substitution is the single commonest route to a tenfold error that we have documented.

On a syringe marked to a hundred units, one unit is a hundredth of a millilitre. That single sentence is the whole of the convention, and almost every arithmetic error in this subject comes from not having it.

Dead space, air and the dose that stays behind

Dead space is the volume held in the needle and hub after the plunger has bottomed out. For a fixed-needle insulin syringe it is very small, of the order of two to seven microlitres. For a detachable needle on a conventional luer fitting it is considerably larger, sometimes exceeding fifty microlitres.

Whether that matters is a question of proportion. At an intended volume of two hundred microlitres, a five-microlitre loss is under three per cent and irrelevant. At an intended volume of twenty microlitres — which a concentrated reconstitution produces — the same loss is a quarter of the dose. This is one of the strongest practical arguments against making a vial up to a very high concentration: it pushes the injection volume down into the range where fixed losses dominate.

Air bubbles interact with the same arithmetic. A bubble displaces solution, so a barrel drawn to eight units containing a one-unit bubble delivers seven units of drug. Small bubbles in a subcutaneous injection are not a safety problem in the way they would be intravenously; they are a dosing problem. Expelling them by tapping the barrel upright and pushing the plunger to the mark is a volumetric correction, not a ritual, and it matters most at exactly the small volumes where people are least inclined to bother.

A short glossary

Unit (U-100): ten microlitres. A volume, not an amount of drug. Concentration: mass per volume, here usually milligrams per millilitre. Dead space: volume retained in needle and hub after full depression of the plunger. Priming: expelling a small volume before dosing, to clear air and confirm flow.

Gauge: needle bore, inversely numbered — higher gauge is thinner. Subcutaneous: into the fat layer beneath the dermis. Intradermal: within the skin itself, which is what an oblique short needle risks. Intramuscular: into muscle beneath the subcutaneous layer.

Lipohypertrophy: thickened subcutaneous tissue from repeated injection, with blunted and variable absorption. Lipoatrophy: localised loss of subcutaneous fat, a different and now rare immune-mediated phenomenon. Bacteriostatic: inhibiting microbial growth, not sterilising. In-use period: the interval after first puncture during which a product remains within specification, established by stability testing.

The distinction between bacteriostatic and sterile, and the distinction between purity and content, account between them for a large share of the confused correspondence this desk receives.

Write the actual diluent volume on the vial. Reconstituting with one volume and then calculating from the volume that was intended is invisible afterwards, because the vial looks identical either way, and three seconds with a marker closes it.

One thing we would like to see changed is trivially achievable. Needles are cheap, and reuse is driven almost entirely by cost and availability rather than by any belief that it is safe. Of every technique failure catalogued above, that is the one most responsive to supply, and the one where the barrier is commercial rather than educational.