Dose Calculation

Equipment

The equipment needed to administer fluids can vary depending on how it is going to be administered. A needle and syringe will be needed to remove a medicine or fluid from a vial. Anytime a practitioner delivers a medication through an intravenous (IV) port they must use a flush to push the drug into the IV tubing or a bag of fluid such as normal saline. When calculating infusions, it is important to use a drip chamber and know how many drops are needed to administer one mL of the fluid. There are two different styles of administration drip sets (gtt/set). One is a macro set that uses larger drops to add to 1 mL. Most macro sets are either 10, 15 or 20 drops to make 1 mL. The other drip set is a micro set, and it either takes 45 or 60 drops to make 1 mL. When giving most medications via continuous infusion, micro drip sets are the preferred method. One easy way to differentiate the two is by looking into the drip chamber. A clinician will see a micro set adapter that looks like a small silver pin coming from the bag end of the chamber. This pin causes the drops to be tiny, hence the term micro set. Some medications that are infused come in a glass vial. If this is the case, make sure to use a drip tubing that has a vent port to allow air into the bottle. Without this air entering the glass vial/bottle, the release of the fluid will eventually cause a suction, and the medication will stop flowing.

Preparation

Preparing a dose calculation is best done with the use of a calculator, particularly in a stressful situation. If not, errors are likely. One wrong decimal place can become a ten-fold error that can cause life-threatening complications for patients. [2] Knowing the three methods of dosing calculations can serve as a check and balance system to verify dose arrived at by calculator use and is recommended to avoid medication errors. An excellent way to prepare for standard drip rates and concentrations is to develop a chart with the drip rates already calculated for ease. [3] Caution should also be used when doing so. With different manufacturers and pricing wars, it is very common for a company to change the vial and concentration without notifying providers. If providers become complacent in this situation, the patient could become injured, and the providers might be liable.

Dose calculations should always be performed before the medicine is mixed if the practitioner is giving an infusion. Doing the math before mixing allows a person to change the numbers to reach a better drip-rate. It is difficult to count 600 gtt/min using a 60 gtt/set but changing the drip set to a ten gtt/set can decrease your rate to 100 gtt/min and make your treatment more manageable.[4][5][6][7]

Technique or Treatment

Volume Infusion

This method can be used when a patient needs a certain amount of IV fluid over an amount of time in minutes. This fluid can be by itself, or it can have medication mixed in the bag. The overall goal is to give the fluid over a particular time in minutes. The answer to the formula will be in drips per min.

(Volume (mL) x (gtt/set)) / Time (min) =  gtt/ min

Example: 250 mL of normal saline over 30 minutes with a ten gtt/set is needed. Plugging these numbers into the formula would tell the clinician that they need to set the drip rate at approximately 83 gtt/min or about 1.5 gtt/sec.  

Example: The clinician places 150 mg of amiodarone into a 100 mL bag of D5W with a ten gtt/set. The clinician is going to deliver the entire 150 mg over 10 minutes; therefore, it is irrelevant that the medicine is in the bag. The goal has become to give the 100 mL bag over 10 minutes. When you are using the formula with (100 mL x 10 gtt/set) divided by 10 minutes, you get 100 gtt/min or about 1.5 gtts/sec.

Bolus

This method is used when the clinician must obtain a specific amount of medicine out of a container. The answer obtained from the formula will be the amount in mL.

(Drug ordered / Drug Available) x  total mL in vial =  mL

Example: The clinician is to deliver 0.2 mg of epinephrine (adrenaline) to a pediatric patient in a bradycardic rhythm. If using a 1:10,000 epinephrine concentration (1 mg in 10 mL), the formula would indicate that they must draw 2 mL from the vial.

Continuous Infusion

When giving a constant amount of medication every minute for an extended period, continuous infusion calculation can be used. Once the clinician calculates the correct drips per minute but wants to change the dose, they can just run the calculation again with the new formula. 

(Drug ordered dose (per min) x (gtt/set) / Total drug available) x total mL in infusion bag = gtt/min

Example: The clinician is treating a hypotensive patient with an epinephrine (adrenaline) infusion at 2 mcg/min. The clinician needs to obtain the bag of fluid and the vial of epinephrine. They use one vial of epinephrine 1:1,000, place it into a 250 mL bag of normal saline (NS), and use a 60 gtt/set. The formula would yield 30 gtt/min.

Example: The clinician is giving dopamine at 5 mcg/kg/min to a 43-year-old man who weighs 100 kg. The clinician has a 500 mL bag of normal saline (NS), 400 mg of dopamine in a vial, and a 60 gtt/set. The formula would yield 37.5 gtt/min. They would, in fact, not be able to obtain 37.5 gtt/min, but one drop every 1 to 1.5 seconds will get close.

Clinical Significance

Knowing these formulas and when to use them can help practitioners to deliver the right amount of medicine to the patient. Continually keeping up with current standards of care as evidence-based practice changes will assist in medication administration safety. Often, clinicians need to think on their feet.

Enhancing Healthcare Team Outcomes

Pharmacists, physicians, pharmacy technicians, and nurses, etc. must know how to calculate medication doses. There are several formulas that can be used. When in doubt, a second opinion should be sought. Double-checking a calculation with another healthcare team member, e.g., nurses and pharmacists should always be considered if there is any doubt regarding the calculation.