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)