Formulas, General

Updated: Apr 16, 2018
Author: Buck Christensen; Chief Editor: Buck Christensen 

General Formulas

The following are commonly used general formulas:[1]

  • Alveolar gas equation[2] : pA O2 = 150 - pa CO2/0.8

  • Minute ventilation[3] = tidal volume x respiratory rate (normal is 4-6 L/min)

  • Tidal volume = alveolar space + dead space

  • Anion gap[4] = [Na+] - ([Cl-] + [HCO3−]) (normal is 12 ± 2)

  • Delta-delta = (anion gap - 12) / (24 - HCO3)

  • Osmolal gap = measured osmoles - calculated osmoles (normal is < 10)

  • Osmoles = (2 x [Na+]) + (glucose/18) + (BUN/2.8) + (ethanol/4.6)

  • Creatinine clearance = (140 – age) x weight (kg) / serum Cr x 72

  • Fractional excretion of sodium = (urine Na / plasma Na) / (urine Na / plasma Na)

  • Corrected calcium = Ca + (normal albumin - albumin) x 0.8

  • Body surface area = Square root of (height [cm] x weight [kg] / 3600)

  • Sensitivity = a / (a + c)

  • Specificity = d / (d + b)

  • Prevalence = (a + c) / (a + b + c + d)

  • Positive predictive value = a / (a + b)

  • Negative predictive value = c / (c + d)

  • Accuracy = (a + d) / (a + b + c + d)