General Formulas
The following are commonly used general formulas [1] :
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Alveolar gas equation [2] : pA O2 = 150 - pa CO2/0.8
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Minute ventilation [3] = tidal volume x respiratory rate (normal is 4-6 L/min)
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Tidal volume = alveolar space + dead space
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Anion gap [4] = [Na+] - ([Cl-] + [HCO3−]) (normal is 12 ± 2)
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Delta-delta = (anion gap - 12) / (24 - HCO3)
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Osmolal gap = measured osmoles - calculated osmoles (normal is < 10)
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Osmoles = (2 x [Na+]) + (glucose/18) + (BUN/2.8) + (ethanol/4.6)
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Creatinine clearance = (140 – age) x weight (kg) / serum Cr x 72
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Fractional excretion of sodium = (urine Na / plasma Na) / (urine Na / plasma Na)
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Corrected calcium = Ca + (normal albumin - albumin) x 0.8
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Body surface area = Square root of (height [cm] x weight [kg] / 3600)
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Sensitivity = a / (a + c)
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Specificity = d / (d + b)
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Prevalence = (a + c) / (a + b + c + d)
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Positive predictive value = a / (a + b)
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Negative predictive value = c / (c + d)
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Accuracy = (a + d) / (a + b + c + d)