Systematic Arterial Blood Gas Interpretation

Overview & definition

The arterial blood gas (ABG) reports oxygenation, acid-base status, and (on most analysers) lactate, electrolytes, and haemoglobin. Interpreting it systematically — in the same order every time — avoids missing a mixed disturbance. The six steps are: (1) oxygenation, (2) pH, (3) the primary disturbance, (4) compensation, (5) the anion gap (albumin-corrected) with the delta gap, and (6) the base excess, the SpO2, and the lactate.^[1][3]

The core normal values: pH 7.35-7.45 and PaCO2 35-45 mmHg at sea level; acidaemia is defined as a pH under 7.35.^[20][28][12] The PaO2 must be read against local reference values — it falls with age and with altitude, so no single number is "normal" for every patient.^[20] A lactate over 2 mmol/L is hyperlactataemia; lactic acidosis is generally defined as a lactate over 4 mmol/L, usually with a pH under 7.35 and a plasma bicarbonate under 20 mmol/L.^[13] The anion gap is normally 8-12 mmol/L.^[12] The "six-step" framework is the construct used here and in the major reviews, which likewise walk a fixed sequence — pH, PaCO2 and bicarbonate, the expected compensation, the anion gap, then the osmolar gap and gap-gap analysis where indicated — because deviations from the expected compensation reveal additional acid-base processes.^[3][6]

^{[20] [28] [12] [13] [21]}

References36ShowHide

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