title: Are arterial and venous samples clinically equivalent for the estimation of pH, serum bicarbonate and potassium concentration in critically ill patients?
abstract: OBJECTIVE
  To assess the comparability of venous and arterial samples for pH, bicarbonate and potassium measurements in critically ill patients.
  
  
  METHODS
  Simultaneous arterial and venous samples from 206 critically ill patients were analysed in duplicate. Coefficients of variation and 95% limits of agreement were calculated for arterial and venous samples. Bland-Altman plots were constructed to assess agreement between sampling sites.
  
  
  RESULTS
  The median (range) of arterial pH, bicarbonate concentrations, potassium concentrations and glucose concentrations were 7.40 (7.01-7.56), 25 (9-41) mmol/l, 4.2 (3.1-6.8) mmol/l and 7.4 (3.0-13.5) mmol/l, respectively. Coefficients of variation for arterial and venous pH were both 0.1%, with bias (95% limits of agreement) of -0.01 (-0.03 to 0.01) for arterial and -0.01 (-0.02 to 0.01) for venous samples. The bias (95% limits of agreement) between arterial and venous samples was 0.03 (-0.02 to 0.08). Coefficients of variation for arterial and venous bicarbonate results were 0.8 and 0.7%, respectively, with bias (95% limits of agreement) of 0 (-0.5 to 0.5) mmol/l for both sample types. The bias (95% limits of agreement) between venous and arterial samples was 0 (-1.3 to 1.3) mmol/l. Coefficients of variation for arterial and venous potassium samples were 0.8 and 1.1%, respectively, with bias (95% limits of agreement) of 0 (-0.1 to 0.1) for both sample types. The bias (95% limits of agreement) between venous and arterial samples was 0.1 (-0.4 to 0.6) mmol/l.
  
  
  CONCLUSIONS
  A venous blood sample, analysed on a blood gas machine, is sufficiently reliable to assess pH, bicarbonate and potassium concentrations in critically ill patients, suggesting that venous sampling alone is appropriate in the management of diabetic ketoacidosis.
authors.affiliation: Department of Acute General Medicine, The John Radcliffe Hospital             Kadoorie Centre for Critical Care Research and Education, The John Radcliffe Hospital, Oxford, UK.
authors.forename: W G
authors.initials: WG
authors.lastname: Herrington
authors.forename: H J
authors.initials: HJ
authors.lastname: Nye
authors.forename: M S
authors.initials: MS
authors.lastname: Hammersley
authors.forename: P J
authors.initials: PJ
authors.lastname: Watkinson
mesh_terms: D000137:Acid-Base Imbalance / Q000097:blood* / Q000209:etiology; D001158:Arteries; D001639:Bicarbonates / Q000097:blood*; D001784:Blood Gas Analysis / Q000379:methods; D016638:Critical Illness; D016883:Diabetic Ketoacidosis / Q000097:blood / Q000150:complications; D005260:Female; D006801:Humans; D006863:Hydrogen-Ion Concentration; D008297:Male; D008875:Middle Aged; D011188:Potassium / Q000097:blood*; D011446:Prospective Studies; D015203:Reproducibility of Results; D014680:Veins
chemical_list: D001639:Bicarbonates; D011188:Potassium