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IV Flow Rate and Infusion Math Made Simple!

Dec 20th, 2022
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IV Flow Rate and Infusion Math

IV flow rate and infusion math often features on the PTCB exam. Whilst it may not feature heavily, candidates should still prepare for this kind of question for the pharmacy technician exam. After all, one question can make all the difference between passing and failing.

As with all types of PTCB math question, the strategy is about breaking things down to the simplest possible level. Then, you can work your way up toward a complete understanding of the subject. Remember, knowing pharmacy math is not just important for the PTCB exam; it’s also important in its application for your professional career. Always learn PTCB math not because it’s featured on the exam but because it’s important to know for your career. That way, you can summon the effort needed to tackle this type of question, and every other type of math question on the PTCB test.

With even a little effort, IV flow rate and infusion math can become some of the easiest questions to answer on the PTCB test. It may not seem like it now but, in time, it will.

Here, we provide a broad overview of the subject; reviewing some sample questions – breaking each question down on a step-by-step level to help you both understand and answer this type of PTCB math question.

What are infusions and why are they needed?

Infusion therapy is widely given to patients for many different reasons.

Infusion therapy means that medicine is delivered to the patient via a needle or catheter – typically via the intravenous rote, direct into veins. This mode of drug administration is particularly helpful when patients cannot take medicines via the oral route. Instead, medicine can be delivered direct into the bloodstream where it can have a more rapid effect.

Remember: it takes time for orally administered drugs to work. They need to bypass enzymes of the oral cavity; acidity of the stomach; and enzymes that flow into the upper intestinal tract; as well as the hefty metabolism of the industrial liver. IV infusions bypass all this traffic – instead, going direct to the bloodstream where they can exert a more rapid effect at the target site of action. For similar reasons infusions are also effective for drugs that cannot withstand the metabolism, acidity, and enzymes of the GI tract. These drugs would get broken down, not making it to the bloodstream. Instead, they must be administered direct into veins. IV pumps are also helpful for patients who need to sustain hydration or in patients that must have their electrolyte levels boosted.

Infusion therapy allows drug to be administered quickly or slowly – depending on the needs of the patient and their condition. For example:

  • a patient having a heart attack or an anaphylactic reaction or a stroke, can be treated quickly with an IV infusion. Time is of the essence in these emergency situations and IV infusions are highly effective.
  • similarly, some IV infusions are administered slowly – with a controlled drop rate – as we often see in chemotherapy, for example.

These are some of the core reasons why IV flow rate and infusion math is tested on the PTCB exam.

Infusions and flow rates play a critical role in the day-to-day therapeutic management of many patients. Pharmacy technicians are expected to understand not only the mechanisms of working this math out, but to do so in an accurate and responsible manner.

This is the foundational duty of every healthcare professional.

IV Infusion and Flow Rate Math Questions

Remember: on the PTCB exam there are 90 questions, only 80 of which are scored. Therefore, the number of PTCB math questions asked is limited – and IV flow rates and infusions may only form a small part of your exam. In fact, it may not feature at all!

But nobody knows what will appear and what won’t appear, and so preparation of all topics is essential.

Below, we review two sample PTCB IV flow rate and infusion questions.

We break each question down – and detail the important elements you should keep in mind when answering these questions. Try to attempt each question first and note the method you are using, before reading the explained answers. Then, compare your method versus our detailed explanation.

Even if you manage to find the correct answer, you may find new ways to understand about how to solve the problem in a more safe and effective manner.

Example 1

A patient receives an infusion of 220 mL/hr of drug from an IV bag that contains 3L. Approximately how many hours will the infusion last?

Each hour that the patient is receiving the infusion, 220 mL of medicine is being administered.

In total, the patient will receive 3,000mL (remember, always convert units!).

Therefore, 3000 mL / 220 mL per hour = 13.6 hours

Units are critical. Whilst this is a more basic example of an infusion question, the principle is core to many more difficult problems (and even many other topics in pharmacy technician math).

By double-checking units are consistent, and by having the knowledge in advance to identify and perform these conversions, it makes the math operations much easier and far more practical. It helps to avoid unnecessary and avoidable mistakes.

Example 2

A 58-year-old patient has been admitted to hospital with a suspected poisoning. An infusion of 1,600 mL is administered to a patient over a 6-hour period.

If the calibration rate is set to 8 gtts/mL, how many drops per minute will the patient receive?

First question: are units consistent?


We are talking about a “6-hour period”, whilst the question is asking us to calculate “drops per minute”. Therefore, the first job is to convert 6-hours to minutes.

6 hours x 60 = 360 minutes

Now over that 360-minute period, the patient receives 1,600 mL. We need to find out how much mL the patient received for each minute of that 360-minute period.

And we do that by simple division:

1,600 mL / 360 minutes = 4.44 mL per minute

So, for the 6-hours that the patient is receiving the infusion, they are receiving 4.44 mL each minute.

The reason we need to perform the above operations is because the question is asking us to calculate “how many drops per minute” the patient will receive.

We are told the calibration rate: it’s 8 drops per mL (gtts is the pharmacy abbreviation for “drops“!).

In other words, for each 1 mL the patient receives, 8 drops is infused into their body (8 drops = 1 mL).

But we have worked out that 4.44 mL is being administered. Therefore, we simply need to perform the following multiplication step:

4.44 mL per minute x 8 drops per mL = 35.5 drops per minute.

Let’s recap:

  • The patient receives an infusion of 1,600 mL over a 6-hour period.
  • We established that this is the equivalent of 4.44 mL for each minute.
  • We are told that there are 8 drops in 1 mL (the set calibration rate for the infusion).
  • Therefore, 4.44mL contains 35.5 drops – administered per minute over the entire 6-hr infusion.

Once you break down what’s happening with IV flow rate and infusion math, the PTCB exam questions become much more manageable.

Take Home Message

IV flow rate and infusion math has a bad reputation.

But as we have shown here, it doesn’t need to be difficult. It’s about breaking the question down to its most fundamental parts and understanding what is going on.

One of the biggest mistakes that pharmacy technicians make is relying on memorizing formulae. This is great, of course, if the formulae are understood and applied correctly. Often, however, formulae are memorized without a clear understanding of how to properly apply these formulae to different PTCB math questions. This can lead to simple and avoidable mistakes.

The two examples we reviewed attempted to break down the questions without relying on formulae. Instead, the focus was on stripping back the question and understanding exactly what’s going on at the most basic, step-by-step level. This is what creates understanding.

By starting here and building up, your knowledge base of IV flow rate and infusion math will grow faster and – more importantly – with a greater sense of understanding of what the question is asking and how you can develop a plan to work out the correct answer: accurately, reliably, and informed. Then, your confidence with this kind of math will grow and – no matter what the PTCB exam throws at you – you’ll have the topic well and truly covered!

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