Essential Pharmaceutical Calculations and Clinical Formulas

Weighing and Measuring Fundamentals

• Percentage Error in Weighing: % error = (sensitivity / amount weighed) × 100

Buffer Solutions and pH Calculations

Calculating pH for Weak Acid Buffers

• Henderson-Hasselbalch Equation: pH = pKa + log₁₀([salt] / [acid])
• [salt] = concentration of the salt (mol/L)
• [acid] = concentration of the acid (mol/L)
• Antilog of x: 10^x (log transferred to the right-hand side becomes antilog)

Calculating Buffer Capacity

• Ka: Dissociation constant
• [H+]: Hydrogen ion concentration
• C: Total buffer concentration in mol/L

Clinical Calculations and Body Metrics

Ideal Body Weight (IBW) Formulas

• Male: 50 kg + 0.9 kg for each cm above 152 cm
• Female: 45.5 kg + 0.9 kg for each cm above 152 cm
• Flow Rate Conversion: To convert mL/min to mL/sec, divide the mL/min value by 60.

Unit Conversions and Molar Calculations

Mass and Volume Shortcuts

• Mass: kg → g → mg → µg → ng → pg
  • Move right: Multiply by 1,000
  • Move left: Divide by 1,000
• Volume: L → mL → µL
  • Move right: Multiply by 1,000
  • Move left: Divide by 1,000
• Equivalency: 1 mL = 1 cm³

Length and Substance Amount

• Length: m → cm → mm → µm → nm
  • 1 m = 100 cm; 1 m = 1,000 mm; 1 mm = 1,000 µm; 1 µm = 1,000 nm
• Moles: mol → mmol → µmol
  • Move right: Multiply by 1,000; Move left: Divide by 1,000

Mole and Temperature Formulas

• Mass Formula: m = CMV (Mass in grams = concentration in mol/L × molecular weight in g × volume in L)
• Temperature Conversions:
  • Celsius to Fahrenheit: (°C × 9/5) + 32
  • Fahrenheit to Celsius: (°F - 32) × 5/9
  • Celsius to Kelvin: K = °C + 273.15

Density, Specific Gravity, and Percentages

Concentration Expressions

• 1% w/v: 1 g in 100 mL of product
• 1% v/v: 1 mL in 100 mL of product
• 1% v/w: 1 mL in 100 g of product
• Percentage to Decimal: Divide by 100
• Calculating Percentage Volume: (y mL / x mL) × 100

Parts Per Million and Ratios

• 1 part per million (ppm): 1 g in 1,000,000 mL or 1 mg in 1,000 mL (1 liter)
• Ratio to Percentage (e.g., 1 in 500):
  • 1 g in 500 mL = x g in 100 mL
  • 0.2 g = 0.2% w/v
• General Percentage Error: (error in reading quantity / quantity measured) × 100%

Posology and Dosage Determination

Calculating Drug Quantity per Dose

• Actual Patient Intake: Determine the amount of drug in a single dose.
• Daily Total: If multiple doses are required, multiply the single dose by the number of doses per day.
• Example:

Body Surface Area (BSA) Calculation

• Mosteller Formula: BSA = W^0.425 × H^0.725 × 71.84
• W = weight in kg; H = height in cm
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Body Cavity Delivery Systems

Suppository and Base Calculations

• Total Weight: Number of suppositories × weight of each
• Drug Amount: (Percentage / 100) × total weight
• Base Amount: Total weight - drug amount
• Weight Distribution:
  • 1 part by weight = 1/5 × calculated drug amount
  • 4 parts by weight = 4/5 × calculated drug amount
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