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�SESSION 04: �DRUG PRESCRIPTION AND DOSAGE CALCULATION�

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Tandabui Institute of Health Sciences and Technology

J.JULIUS

(BSN(HONS), RN)

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LEARNING OBJECTIVES

  • At the end of this session you should be able to do the following
  • Define drug prescription and dose calculation
  • Identify the components of a drug prescription
  • Calculation of a drug/medicine dosage

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WHAT IS……?

  • Drug prescription?
  • Dose calculation?

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DEFINITION

  • Drug prescription (drug order)
  • Is an instruction written by a medical practitioner that authorizes a patient to be issued with a medicine or treatment
  • Medication order may be prescribed by:
    • Physician
    • Dentist
    • Advanced Practice Registered Nurse (APRN)

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DEFINITION

  • Drug dosage calculation
  • This is a method for calculating medication (drug dosages)
  • The drug quantities of medicines are not always indicated on the prescription

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Components of a Drug Order (Prescription)

  • Date & time the order is written
  • Drug name (generic preferred)
  • Drug dosage
  • Frequency and duration of administration
  • Any special instructions for withholding or adjusting dosage
  • Physician or other health care provider’s signature or name
  • Signature of licensed practitioner

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  • Nurse must check the following in a prescription before dispensing medicine
  • Check medication order is complete & legible
  • know general purpose/action
  • Dosage
  • Route of drug
  • Compare drug card with drug label three times
    • At time of contact with drug bottle/ container
    • Before pouring drug
    • After pouring drug

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Abbreviations used in a prescription

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  • q.i.d means 4 times per day ( = every 6 hours)
  • t.i.d means 3 times per day (= every 8 hours)
  • b.i.d means 2 times per day (= every 12 hours)
  • o.d means 1 times per day (= every 24 hours)
  • Dose based on body weight

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CALCULATION OF DRUG DOSAGE

  • Drug dosage calculations are required when the amount of medication ordered (or desired) is different from what is available on hand for the nurse to administer.
  • So, it is the responsibility of the dispenser to calculate the required amount of the drug to be dispensed.
  • The quantities are expressed in various ways and it often depends on the type of drug that has been prescribed:

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    • As weight (e.g. 50mg, 5g) or volume (e.g. 5ml, 50ml or 300ml)
    • As number of units of the dosage form (e.g. 40tablets or 20capsules)
    • As number of original packs from the manufacturer e.g. 2 boxes
    • Other drugs are being packed in unit packs which contain a full course of treatment (e.g.for 1 week or 4 weeks, such as ARVs).
    • The quantity of a drug to be calculated is often based on the duration of treatment and daily dosage

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  • Duration of the treatment may be indicated:
    • 5/7 = for 5 days (out of 7 days in a week)
    • 2/52 = for 2 weeks (out of 52 weeks in a year = 14 days)
    • 1/12 = for 1 month (out of 12 month in a year)

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TABLET AND CAPSULE CALCULATION

  • To calculate the number of tablets or capsules or amount to be taken at each dose, applying the following formula:
  • Dose = Desired amount/Amount on Hand x Quantity

X= D/H x Q

Where;

D- Desired amount (this is the dose ordered/prescribed)

H- Amount on hand (this is the dosage you have on your Hand per Quantity)

Q – Quantity (read containers label); Therefore Q = 1 tablet or capsule

X- Dose (this is the amount of drug to be administered)

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Tablet and capsule calculation

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  • Note: When medication is given in tablets, the QUANTITY = 1 since the amount of medication available is specified per (one) tablet.
  • Example 1: Toprol XL, 50 mg PO, is ordered. Toprol XL is available as 100 mg per tablets. How many tablets would the nurse administer?

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SOLUTION

  • Amount desired (D) = 50 mg
  • Amount on hand (H) = 100 mg tablets
  • Quantity = 1
  • Dose=?

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Example 2:

  • 1200 mg of Klor-Con is ordered. This medication is only available as 600 mg per tablet. How many tablets should the nurse give?

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  • Amount desired (D) = 1200 mg
  • Amount on hand (H) = 600 mg
  • Quantity = 1
  • Dose=?

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LIQUID CALCULATION

  • Same formula Desired dose (amount) = ordered Dose amount/amount on Hand x Quantity(D/H x Q = x)
    • D-Desired amount
    • H- Amount on hand
    • Q- Quantity
    • X- Dose
  • In these types of calculations, the volume available on hand is the QUANTITY.

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EXAMPLE 1:

  • A provider requests lorazepam 4 Mg IV Push for a patient in severe alcohol withdrawal. The clinician has 2 mg/mL vials on hand. How many milliliters should he or she draw up in a syringe to deliver the desired dose?

Solution

    • D/H x Q = X OR

Desired dose (amount) = Desired Amount /amount on Hand x Quantity

      • Desired amount =4mg
      • Amount on hand=2mg
      • Quantity=1 ml

4/2x1ml=2mls

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EXAMPLE 2:

  • Dilantin-125 is available as 125 mg/5 mL. Dilantin-125, 0.3 g PO, is ordered. How much should the nurse administer to the patient?

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Solution

step 1: Given data

  • Desired amount (D) = 0.3 g
  • Amount on hand (H) = 125 mg
  • Quantity = 5 mL

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  • Step 2: Convert 0.3 g to mg (since the ordered dose is in grams but the drug is available on hand in milligrams).
  • 0.3 g x 1,000 mg/g = 300 mg

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EXAMPLE 3:

  • Furosemide is available as 40 mg in 1 mL. 10 mg is ordered to be administered through an IV. What amount of furosemide should the nurse administer?

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SOLUTION

  • Amount desired (D) = 10 mg
  • Amount on hand (H) = 40 mg
  • Quantity = 1 ml
  • Dose =?

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DOSAGE CALCULATIONS BASED ON BODY WEIGHT

  • Dosage calculations based on body weight are required when the dosage ordered and administered is dependent on the weight of the patient.
  • For example, many pediatric drugs are ordered and given per weight (usually in kg).
  • Dosage calculations based on body weight are calculated in two main stages.

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  • Stage 1: Using the formula below, calculate the total required dosage based on given the body weight.

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  • Weight (kg) x Desired amount(per kg) = Y (Required Dosage)

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  • Stage 2: Apply the D/H x Q formula to calculate the actual amount of medication to be administered.

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  • Example 1: Medrol 4 mg/kg is ordered for a child weighing 29.45 kg . Medrol is available as 500 mg/4mL. How many milliliters of medication must the nurse administer?

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  • Weight (kg) x Dosage Ordered (per kg) = Y (Required dosage)
  • 29.45 kg x 4 mg/kg = 117.8 mg
  • Therefore, the required dosage of medication is 117.8 mg.

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CALCULATION OF INTRAVENOUS DRIP RATES

  • In these types of calculations, for a given volume, time period, and drop factor (gtts/mL), the required IV flow rate in drops per minute (gtts/min) is calculated.
  • gtts, is an abbreviation meaning drops from the Latin "guttae", which means drops 
  • The answer is usually reported as dpm, gtts/min or drops/min
  • Note: Since a fraction of a drop is not possible to give to a patient, it is usual to round the answers to the nearest whole number.

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CALCULATION OF INTRAVENOUS DRIP RATES

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Formula

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EXAMPLE 1:

  • Calculate the IV flow rate for 250 mL of 0.5% dextrose to be administered over 180 minutes. The infusion set has drop factor of 30 gtts/mL.

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  • Volume: 250 mL
  • Time: 180 min
  • Drop factor: 30 gtts/mL

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EXAMPLE 2

  • The infusion set is adjusted for a drop factor of 15 gtts/mL. Calculate the IV flow rate if 1500 mL IV saline is ordered to be infused over 12 hours.

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  • Volume: 1500 mL
  • Time: 12 hours
  • Drop factor: 15 gtts/mL

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  • First, Convert 12 hours into minutes.

12h x 60 min/h = 720 min

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