$A-a gradient=PAO2-PaO2PAO2=FiO2(Patm-PH2O)-(PaCO2R)$

PAO2 = Alveolar partial pressure of O2 [mm Hg]

PaO2 = Arterial partial pressure of O2 [mm Hg]

FiO2 = Fraction of inspired O2 (0.21 in room air)

Patm = Atmospheric pressure (about 760 mm Hg at sea level)

PH2O = Partial pressure of water vapor (about 47 mm Hg at 37°C)

PaCO2 = Arterial partial pressure of CO2 [mm Hg]

R = Respiratory quotient, reflecting basal metabolic rate describing amount of CO2 production for a given O2 consumption (ranges from 0.7 to 1; usually 0.8 is used)

• Measure of the efficiency of the oxygenation of blood

• The normal A-a gradient varies with age. A helpful calculation is [age(in years)/4] + 4

• Normal A-a gradient ranges from 5–10 mm Hg when breathing room air and 20–65 mm Hg when on 100% O2. A higher A-gradient reflects impaired transfer of oxygen from the alveoli to the peri-alveolar capillaries

• Gradient is affected by V/Q or diffusion abnormalities but unaffected by hyperventilation or hypoventilation.

• If PaO2 corrects with supplemental O2, consider asthma or other conditions associated with V/Q mismatch.

• If PaO2 does not correct with supplemental O2, consider shunt.

## ABSOLUTE NEUTROPHIL COUNT

$ANC=WBC×(PMN100+Bands100)$

WBC = White blood cell count [cells/mm3]

PMN = Percentage of polymorphonuclear neutrophils

Bands = Percentage of band neutrophils

• Neutropenia is defined as an ANC less than 1500 cells/mm3.

• Severe neutropenia (ANC <500 cells/mm3) is associated with a high risk of infection.

## ANION GAP (SERUM)

$Serum AG=[Na+]-([Cl-]+[HCO3-])$

[Na+] = Serum concentration of sodium ions [meq/L]

[Cl] = Serum concentration of chloride ions [meq/L]

[$HCO3−$] = Serum concentration of bicarbonate ions [meq/L]

• Aids in classifying metabolic acidosis

• The normal anion gap is 8–12 meq/L.

• An elevated anion gap is caused by an increase in unmeasured anions, which may be any of the following:

“MUD PILES”: methanol; uremia; diabetic ketoacidosis; paraldehyde or phenformin; iron, infection, or isoniazid; lactic acidosis; ethanol or ethylene glycol; and s...

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