United States
Hong KongIV flow rates
Flow rate is volume divided by time. Where the answer is wanted in drops per minute, multiply the rate in mL per minute by the drop factor of the administration set.
Two related calculations, and the second is the first with one extra multiplication.
Rate in mL per hour
Total volume divided by total time. A 1,000 mL bag over 8 hours is 125 mL per hour.
Read the units the question wants. Per hour and per minute are the same calculation with a factor of 60 between them.
Rate in drops per minute
Convert the rate to mL per minute, then multiply by the drop factor of the set, which is given in drops per mL.
At 125 mL per hour, that is 125 divided by 60, which is about 2.08 mL per minute. With a set delivering 15 drops per mL, that is about 31 drops per minute.
It is given in the question and it changes the answer entirely. Two identical infusions running through different administration sets have different drop rates, and a candidate who assumes a standard factor rather than reading the one supplied gets a plausible wrong answer.
Running it backwards
A question may give the rate and ask how long a bag will last, or how much volume is needed for a given duration. Same relationship, rearranged.
Volume equals rate times time. Time equals volume divided by rate.
Drops are whole numbers
You cannot deliver a fraction of a drop, so a drops-per-minute answer is rounded. Check what the options look like before deciding how precisely to work.
The order that avoids errors
- Write down volume, time and drop factor separately
- Get to mL per minute
- Multiply by the drop factor only if drops are wanted
- Check the scale - a rate of thousands of drops a minute is wrong
Common questions
How do I calculate an IV flow rate?
Total volume divided by total time. A 1,000 mL bag over 8 hours is 125 mL per hour.
How do I get drops per minute?
Convert to mL per minute, then multiply by the drop factor of the set.
What is the drop factor?
Drops per mL for the administration set. It is given in the question and it changes the answer.
Should drops be rounded?
Yes. You cannot deliver a fraction of a drop.
How do I find how long a bag lasts?
Volume divided by rate. It is the same relationship rearranged.