Syringe Driver Rate Calculator
Syringe Driver Rate Calculator
A continuous subcutaneous infusion made up to a final volume and run over a set period, converted into the number you actually set on the device — in mL per hour for a T34-type pump, in mm per hour for a Graseby MS16A, or in mm per 24 hours for a Graseby MS26. Those last two differ by a factor of exactly 24 for the same infusion, confusing them has killed people, and this page prints all three settings every time so that the convention can never be assumed. It computes a rate from a prescription; it does not decide what to give.
This calculates a rate or a volume from a prescription that has already been written. It does not decide what to give. Check every figure against the prescription and your local policy, and have it independently checked before administration.
Rate to set, in the convention your driver actually uses
A 20 mL syringe made up from 6 mL of drug plus diluent, to run over 24 hours, fluid column measured at 60 mm, checked at 8 hours elapsed, on a pump set in mL/h
One rate, three ways of writing it, and the factor of 24 between two of them
- V_final
- the volume the syringe is made up to, in mL. The volume of drug plus the volume of diluent
- T_hours
- the period the syringe is to run over, in hours. 24 for the usual continuous subcutaneous infusion
- L_measured
- the length of the fluid column in the barrel, in millimetres, measured against the driver’s own scale with the syringe that is going to be used. Not derived from the volume, because barrel diameter varies between manufacturers
- the factor of 24
- the MS26 setting is exactly 24 times the MS16A setting for the same infusion, for every volume, every syringe and every period, because the only difference between the two devices is whether their dial means per hour or per day. That is what makes the confusion so dangerous: both numbers are plausible, both fit the dial, and neither device can tell you which one you meant
- mL per mm
- V_final / L_measured for your own syringe. Worth knowing because it converts a millimetre setting back into a volume rate, which is the only way to compare a Graseby setting against a prescription written in mL/h
Worked example
A 20 mL syringe made up from 6 mL of drug plus diluent, to run over 24 hours, fluid column measured at 60 mm, checked at 8 hours elapsed, on a pump set in mL/h
Diluent first: 20 − 6 = 14 mL of diluent to reach a 20 mL final volume. The rate does not depend on that split, only on the 20 mL.
On a pump set in millilitres per hour the rate is 20 / 24 = 0.83 mL/h. Nothing else is needed: the pump knows the syringe it has been told about and drives the plunger as far as it must to deliver 0.83 mL in an hour.
On a Graseby MS16A the rate is a LENGTH per hour: 60 mm / 24 h = 2.5 mm/h. Note what has happened to the 20 mL — it has disappeared from the calculation entirely, replaced by the 60 mm. That is the whole character of a millimetre-based driver.
On a Graseby MS26 the rate is a length per DAY: 60 mm / 1 day = 60 mm per 24 h. For a 24-hour infusion the setting is just the measured length, which is convenient and is also the trap: it teaches people that the setting is the length, and the next time the period is 12 hours the setting is 120, not 60.
Here is the harm, exactly. 60 and 2.5 are both numbers you can put into a two-digit dial. Set the MS26's 60 on an MS16A and the driver runs at 60 mm per hour instead of 2.5 — twenty-four times too fast — and the 20 mL syringe empties in 60 minutes instead of 24 hours. In a palliative continuous subcutaneous infusion that syringe is a day's worth of opioid, and it goes in within the hour. The Medical Devices Agency issued hazard notices about this in 1994 and 1995 and required the two models to be colour-coded with their units in bold on the control panel; the National Patient Safety Agency issued a rapid response report on 16 December 2010 requiring a transition away from rate-in-millimetre drivers. Patients had already died.
The reverse error is quieter and still serious. Set the MS16A's 2.5 on an MS26 and the device runs at 2.5 mm per day: the syringe lasts 576 hours, twenty-four days, and the patient receives about a twenty-fourth of what was prescribed. Nothing alarms, nothing beeps, and the first sign is uncontrolled symptoms.
The checking row: at 8 hours of a 24-hour infusion, a third of the way through, 20 × 8/24 = 6.67 mL should have gone in and 13.33 mL should be left. Against a 60 mm column that is 40 mm of fluid still in the barrel. If the barrel says something else, the rate that was set is not the rate that was prescribed, and the arithmetic above is where to start looking.
The same infusion, written three ways
| Device | What its dial means | Setting for 20 mL over 24 h at 60 mm | Setting for the same 20 mL over 12 h at 60 mm |
|---|---|---|---|
| McKinley T34 / BD BodyGuard T34 | millilitres per hour | 0.83 | 1.67 |
| Graseby MS16A (blue) | millimetres per hour | 2.5 | 5.0 |
| Graseby MS26 (green) | millimetres per 24 hours | 60 | 120 |
What happens when the convention is wrong, for a 20 mL syringe over 24 hours
| Intended | Set instead | Actual rate | Syringe empties in | Dose delivered in 24 h |
|---|---|---|---|---|
| MS16A at 2.5 mm/h | 60, the MS26 number | 60 mm/h, 24 times too fast | 1 hour | 24 hours’ worth in the first hour |
| MS26 at 60 mm/24h | 2.5, the MS16A number | 2.5 mm per 24 h, 24 times too slow | 576 hours | about one twenty-fourth of the prescription |
| T34 at 0.83 mL/h | 2.5, read as mL/h | 2.5 mL/h, 3 times too fast | 8 hours | three days’ worth in a day |
| T34 at 0.83 mL/h | 60, read as mL/h | 60 mL/h, 72 times too fast | 20 minutes | the whole syringe inside half an hour |
Millilitres per hour, millimetres per hour, millimetres per 24 hours — and why the device has to be identified first
A syringe driver does not know what is in the syringe. It advances a plunger, and the only question is what its rate setting means. Modern ambulatory pumps — the McKinley T34 and the BD BodyGuard T34 that replaced it are the ones most people will meet — are set in millilitres per hour. You tell the pump the syringe brand and size, it works out the barrel geometry, and the rate you dial is a volume rate. The older Graseby MS series does none of that. It is set in millimetres: the plunger advances a fixed distance per unit time, and how much fluid that distance represents depends entirely on the diameter of the barrel in front of it.
There are two millimetre conventions, not one, and the difference is a factor of 24. The Graseby MS16A, which is blue, is set in millimetres per hour. The MS26, which is green, is set in millimetres per 24 hours. For a 24-hour infusion the MS26 setting is simply the measured length of the fluid column, and the MS16A setting is that length divided by 24. A 60 mm column is 60 on one device and 2.5 on the other. Both are valid two-digit settings. Both look entirely reasonable on a chart. Setting the MS26 number on an MS16A delivers a day’s infusion in an hour.
This is not a theoretical hazard. The Medical Devices Agency issued hazard notices in 1994 and 1995 drawing attention to the potential for confusion between the two models, after patient deaths; the mitigation was to colour-code them and print the units in bold on the control panel. In December 2010 the National Patient Safety Agency issued a rapid response report requiring organisations to plan a transition to ambulatory syringe pumps with additional safety features. A Department of Health and Social Care review later set out the whole sequence. The devices are long out of production and most services have replaced them, but they are durable, they are cheap, and they turn up — in hospices, in community kit bags, in countries where replacement was never funded. A calculator that assumes a mL/h pump because that is what is usually in front of you is the same assumption that the alerts were written about.
The millimetre case needs a measurement, not a lookup. Because a Graseby meters length, its setting depends on the barrel diameter, and barrel diameter varies between manufacturers for the same nominal syringe volume. The fluid length for 20 mL is not a constant; it is 4V/πd² with d the internal diameter of that particular barrel. So the input this page asks for is the measured length of the fluid column, taken with a ruler against the scale on the driver, with the syringe that is actually going to be used. The row showing millilitres per millimetre is that relationship for your own syringe, derived from your own two numbers, and it is the only honest way to translate between a millimetre setting and a volume rate.
What this page does and does not do. It takes a prescription that has already been written — these drugs, this final volume, over this period — and converts it into the number to set on the device, in all three conventions at once so that the convention cannot be assumed. It gives the volume that should have been delivered by any elapsed time, which is the standard running check against what is left in the barrel. It does not choose drugs, it does not choose doses, it does not check compatibility and it does not know whether the mixture is stable for the period entered. If you need to dilute the reconstituted contents further before they go in, the C1V1 = C2V2 dilution calculator handles that part.
Frequently asked questions
How do I tell which convention my driver uses?
Look at the device, not at the chart. A T34-type pump has a digital display, is programmed by entering a volume and a duration, and shows a rate in mL/h. A Graseby MS16A or MS26 is a plain box with a rotary rate setting of two digits, a scale printed along its body in millimetres, and its units stated on the control panel — the colour coding and the bold units were added specifically so that this question could be answered at a glance. The MS16A is blue and reads millimetres per hour; the MS26 is green and reads millimetres per 24 hours. If you cannot identify the device with certainty, do not set it.
Why does the page want a measured length rather than the syringe size?
Because a millimetre-based driver meters distance, and the volume in a millimetre of barrel depends on the barrel’s internal diameter, which differs between manufacturers for the same nominal syringe volume. The relationship is L = 4V/πd², so a 10 per cent difference in diameter is a 21 per cent difference in length for the same volume — far more than enough to matter. Measuring the fluid column against the driver’s own scale removes the question entirely and takes a few seconds. It is also what the device instructions ask for.
Does the drug volume affect the rate?
No. The rate depends only on the final volume and the period, or on the measured length and the period. The drug volume is there so the page can tell you how much diluent to add to reach the final volume, which is the other half of making the syringe up. It is worth entering accurately for that reason, and because a drug volume that exceeds the final volume means the syringe cannot be made up as prescribed, which the page will tell you.
The volume left in the barrel does not match what the page says. What now?
Stop and work out which number is wrong before changing anything. More fluid left than expected means the device has been running slower than prescribed: a wrong setting, a paused or alarmed device, a battery, or an occlusion. Less fluid left means it has been running faster, which is the more urgent direction. Compare the setting on the device against the figure for its own convention on this page, and check the period that was used. A mismatch of a factor of 24 in either direction is the Graseby confusion and should be treated as an incident.
Can I use this page for an intravenous infusion?
The arithmetic for a volume over a period is the same, but the context is not, and the three conventions on this page are syringe-driver conventions. For a gravity drip set, an infusion pump, or the remaining time on a running bag, the companion pages in this group are the right ones. This page is about a syringe made up to a final volume and run over a set period by a syringe driver, which is overwhelmingly a continuous subcutaneous infusion.
Why show all three settings when I have only told you one device?
Because the hazard this page exists for is not getting the arithmetic wrong, it is doing correct arithmetic in the wrong convention. If the only number on the screen were the one for the convention you selected, a mis-selection would be invisible. Showing 0.83, 2.5 and 60 side by side means that a wrong selection shows up as a number that does not look like the one on the device, and that the factor of 24 between the two Graseby settings is in front of you every time rather than in a footnote.
Does this page decide the dose or the drug combination?
No, and it will not be made to. It takes a prescription that already exists and converts a volume and a period into a device setting. It has no view on which drugs go in the syringe, in what doses, whether they are compatible, or how long the mixture is stable — all of which are decisions for the prescriber and the pharmacist, and none of which can be inferred from the four numbers this page is given.
Related calculators
References
- Department of Health and Social Care, Review of syringe driver safety actions, for the sequence of alerts and the mechanism of the hazard: the MS16 and MS16A were set in millimetres per hour and the MS26 in millimetres per 24 hours, “the potential for confusion between the two types of syringe drivers which could result in inappropriate infusion rates leading to over-infusion and there had been patient deaths as a result”. Two Medical Devices Agency hazard notices in 1994 and 1995 required the models to be colour coded and their units stated in bold on the control panel; the National Patient Safety Agency rapid response report of 16 December 2010 required a transition plan within a year and a move to ambulatory syringe drivers with additional safety features within five. Cited, not reproduced.
- The MS16A and MS26 rate rules used on this page — MS16A rate = fluid length in millimetres divided by the infusion time in hours; MS26 rate = fluid length in millimetres divided by the infusion time in days — were taken from a hospital Graseby syringe driver procedure and checked against its own worked example (a 48 mm infusion over 24 hours gives 48 on an MS26 and 02 on an MS16A). That example is the identity this page relies on: the MS26 setting is exactly 24 times the MS16A setting, independent of volume, syringe and period.
- The millilitre-per-hour convention is that of the McKinley T34 and its successor the BD BodyGuard T34, the ambulatory syringe pumps named in current NHS and Irish paediatric syringe-pump guidelines for continuous subcutaneous infusion. These are programmed with a volume and a duration and display a rate in mL/h; the barrel geometry is handled by the pump once the syringe brand and size are selected, which is why this page needs no length for that option.
- The barrel-diameter argument is geometry, derived here rather than quoted: the length of a fluid column of volume V in a cylinder of internal diameter d is L = 4V/πd², so length varies as the inverse square of the diameter and a 10 per cent diameter difference changes the length for a fixed volume by 21 per cent. This page therefore takes the measured length as an input and derives millilitres per millimetre from it, rather than shipping a table of syringe dimensions that would be wrong for any syringe not in the table.
- No part of this page reproduces a drug chart, a dose table, a compatibility table or a stability table, and nothing on it selects a drug or a dose. It converts a prescribed volume and period into a device setting, in three stated conventions. Drug selection, dose, compatibility and stability for a continuous subcutaneous infusion are decisions for the prescriber and the pharmacist and are deliberately absent.
CalcEngines health calculators are for education and for checking arithmetic that has already been decided elsewhere. They are not medical advice, they do not decide what to give, and they do not replace the judgement of a doctor, nurse, midwife or dietitian who knows the person in front of them. Every figure depends on the values you enter and on the assumptions stated on the page — check it against the prescription, the product label and your local policy before acting on it.