Classification of acid-base disorders by simultaneous arterial and venous blood gas
Mohamed Sheta, MD1, Thomas Hostetter, MD2, Paul Drawz, MD, MHS, MS1
1. Division of Renal Diseases & Hypertension, University of Minnesota, Minneapolis, MN
2. Department of Medicine, Case Western Reserve University, Cleveland, OH
Background: An arterial blood gas (ABG) is the gold standard for defining acid-base disorders.
A relatively high correlation between arterial and venous pH has led to the routine use of venous
blood gases (VBG) to classify acid-base disorders at some institutions. However, the rate of
agreement between acid-base diagnoses based on arterial and venous blood gasses is
unknown.
Methods: All patients at the University of Minnesota Medical Center with a VBG within 60
minutes of an ABG between November 2010 and November 2013 were included in this
retrospective study. Patients’ acid-base status was categorized based on pH (<7.35 or > 7.45),
pCO2 (< or >40), and bicarbonate (< or >24) from both the ABG and VBG (after applying a
correction factor of 0.05 for pH and 5 for pCO2 to VBG values). Primary analyses focused on
patients with mild-moderate acid-base disturbances (ABG pH 7.25 to 7.50).
Results: Primary analyses included 1604 patients with a mild or moderate acid–base
disturbance. Concordance between diagnoses from arterial blood gases and those from venous
blood gases in patients with a normal pH was excellent at 82%. For patients with an abnormal
pH from arterial blood gases, concordance was only 46%. Interestingly, concordance was
higher for acidotic conditions (57 to 79%). For alkalotic states, the range was 36 to 56%. Results
were similar in analyses including patients regardless of ABG pH and when restricting analyses
to patients with a VBG within 15 minutes of an ABG.
Conclusions: These data suggest that in categorizing the acid–base status of patients,
diagnoses from arterial blood gases differ substantially from those from venous blood gases.
Thus, the measurement of arterial blood gases is necessary in the accurate diagnosis of acid–
base disorders.