While reviewing a medication chart, the nurse noticed that the injection of ampicillin is supposed to be 500 mg OD. The available stock strength is 1 gram/5 mL. How much volume of drug should the nurse administer to the patient?

1. A

   10 ml

2. B

   20 ml

3. C

   2.5 ml

4. D

   5 ml

Appeared in: DSSSB 13 August 2024

Explanation

• The core task is to calculate the correct volume of medication based on the prescribed dose and available stock concentration.
• A critical first step is unit conversion. The prescribed dose is 500 mg, while the stock is 1 gram. You must convert 1 gram to 1000 mg to ensure consistency.
• The standard formula for liquid medication calculation is (Desired Dose / Available Dose) × Volume.
• Plugging in the values: (500 mg / 1000 mg) × 5 mL simplifies to 0.5 × 5 mL.
• The final calculated volume to be administered is 2.5 mL.

Why Other Options Were Wrong

• Option A: Administering 10 mL would deliver 2000 mg of ampicillin, which is four times the prescribed dose of 500 mg.
• Option B: Administering 20 mL would deliver 4000 mg of ampicillin, an amount eight times the prescribed dose, leading to a severe overdose.
• Option D: Administering 5 mL would deliver the entire 1000 mg (1 gram) from the vial, which is double the prescribed dose of 500 mg.

Related Visual

Clinical Relevance

• Nursing practice connection: Use the key finding related to Medication dosage calculation using the formula method and unit conversion to guide bedside assessment, documentation, and the next nursing action.
• Accurate medication calculation is a fundamental nursing skill and a critical component of patient safety. Errors can lead to ineffective treatment (underdosing) or toxicity (overdosing).
• This skill is directly related to the 'Six Rights of Medication Administration,' specifically the 'Right Dose.'.
• Many healthcare facilities have a policy requiring a second nurse to independently double-check calculations for high-risk medications or any dose calculation to prevent errors.

How to Approach the Question

• First, identify the three key pieces of information: the desired dose (what is prescribed), the available dose (what you have on hand), and the volume the available dose is in.
• Check if the units for the desired dose and available dose are the same (e.g., both are in mg). If not, convert one to match the other. In this case, convert 1 gram to 1000 mg.
• Apply the standard dosage calculation formula: (Desired Dose / Available Dose) × Volume.
• Substitute the values into the formula: (500 mg / 1000 mg) × 5 mL.
• Perform the calculation to find the final volume to administer.
• As a final safety check, mentally estimate the answer. Since 500 mg is half of 1000 mg, you should expect to give half of the 5 mL volume, which is 2.5 mL. This confirms your calculation.

Concept Tested & Keywords

• Concept Tested: Medication dosage calculation using the formula method and unit conversion.
• Stem keywords: medication chart, ampicillin, 500 mg, 1 gram/5 mL, volume of drug
• Lead-in keywords: How much

Question ID

QU5t2O6q7Tpe5_SsIMmuDB

Copy ID

Reference Book

E6 Nursing Fundamentals Potter Perry 12e Part 3 pp. 141-143, 142-144

E6 Pharmacology Nurses Shanbhag 3e p. 55-57