# Formulas, General

Updated: Apr 16, 2018
• Author: Buck Christensen; Chief Editor: Buck Christensen  more...
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## 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)