Learn: Dosage Calculations

Overview

Welcome! In this session, we're diving deep into the world of dosage calculations—a vital skill for every nurse. You'll learn not just how to perform the calculations, but also why certain methods are used in specific situations, especially relating to pharmacological and parenteral therapies. Together, we'll explore key concepts, step-by-step strategies, and common pitfalls so you can approach medication math with confidence and accuracy.

Concept-by-Concept Deep Dive

Units of Measurement and Conversions

What it is

Medication dosages and fluid volumes are measured using various units—milliliters, grams, liters, tablespoons, cubic meters, and more. Understanding how to convert between these units is foundational for safe medication administration.

Key Components

• Metric System: The most common system in healthcare. Units include milligrams (mg), grams (g), liters (L), and milliliters (mL).
• Household Units: Sometimes used for oral medications. Examples are teaspoons (tsp) and tablespoons (tbsp).
• Conversion Factors: Essential for translating between systems (e.g., mg to g, mL to L, or tablespoons to milliliters).

Step-by-Step Reasoning

1. Identify the units given and required.
2. Recall or reference the appropriate conversion factor (e.g., 1 tablespoon = X mL).
3. Set up a dimensional analysis equation to cancel out unwanted units.
4. Carry out the calculation, double-checking for logical consistency.

Common Misconceptions

• Mismatching units: Not converting all values to the same system before calculating.
• Using incorrect conversion factors: For example, confusing teaspoons and tablespoons.
• Forgetting to move the decimal: Especially when converting mg to g or mL to L.

Weight-Based and Surface Area-Based Dosing

What it is

Not all patients metabolize drugs the same way. For special populations—especially pediatric patients—dosages are often calculated based on weight (kg) or body surface area (BSA), not just age or generic adult dose.

Key Components

• Why Weight-Based? Children have unique pharmacokinetics; dosing per kilogram ensures safety and efficacy.
• Body Surface Area (BSA): Used for certain drugs (e.g., chemotherapy), as it more accurately reflects metabolic mass.

Calculating BSA

There are several formulas, the most common being the Mosteller formula:

BSA (m²) = √[ (height (cm) × weight (kg)) / 3600 ]

Step-by-Step Reasoning

1. Weigh the patient and record the weight in kilograms.
2. If required, measure height for BSA calculations.
3. Apply the formula specified for the drug or context.
4. Double-check calculations, especially for decimal placement.

Common Misconceptions

• Confusing pounds and kilograms: Always convert to kilograms for calculations.
• Using age instead of weight/BSA for dosing: This can lead to under- or overdosing.

IV Flow Rate Calculations

What it is

Calculating how fast an IV fluid or medication should be administered, often in mL/hour or drops/min (gtts/min).

Key Components

• Drop Factor: The number of drops (gtts) per mL, based on tubing used.
• Ordered Flow Rate: Physician’s order, usually in mL/hour.

Calculation Recipe

• Drops per minute:

  (Volume to infuse per hour × Drop factor) / 60 = Drops per minute

• mL per hour:

  Total volume to infuse (mL) / Total time (hours) = mL/hour

Common Misconceptions

• Mixing up drop factor tubing: Not all IV sets are the same.
• Forgetting to convert hours to minutes: This skews the calculation.

Medication Dosage Formulas and "Desired Over Have"

What it is

The standard formula for calculating how much of a medication to give when you know the "desired" dose and the "have" (what’s available).

Formula Structure