Formulas, General 

Updated: Dec 12, 2014
  • Author: Buck Christensen; Chief Editor: Buck Christensen  more...
  • Print
Sections

General Formulas

The following are commonly used general formulas: [1]

  • Alveolar gas equation: p A O 2 = 150 - p a CO 2/0.8
  • Minute ventilation = tidal volume x respiratory rate (normal is 4-6 L/min)
  • Tidal volume = alveolar space + dead space
  • Anion gap = [Na +] - ([Cl -] + [HCO 3 ]) (normal is 12 ± 2)
  • Delta-delta = (anion gap - 12) / (24 - HCO 3)
  • 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)