Objectives To demonstrate the equivalence between blood collection methods using direct venous puncture (DVP) and a peripheral venous catheter or cannula (PVC).

Design and setting A cross-sectional study of simple crossover design with within-subject measures carried out between October 2011 and May 2012 at a regional hospital in Spain.

Participants 272 patients aged 18 or older hospitalised or admitted to the short-stay unit (SSU) who required laboratory testing and PVC to administer saline solution, intravenous fluid therapy and/or intravenous medication. Excluded were those with PVC collection time exceeding 20 s, difficulty of venoclysis, or who presented with arteriovenous fistula, language difficulties, in critical condition or altered consciousness with no family to consent.

Primary and secondary outcome measures 18 variables were recorded for DVP and PVC, along with age, sex, diagnosis, vein location for DVP, location of the PVC, PVC calibre, saline syringe, intravenous fluid therapy, medication, haemolysis and clotted blood during DVP or PVC collection. Univariate analysis, Pearson's product-moment correlation coefficient (r), Lin's concordance correlation coefficient (r_c) and Bland-Altman's 95% agreement interval were provided.

Results Included in the study were 272 patients, primarily aged 65 or older (80.9%), males (52.6%) and receiving intermittent medication (43.4%). Values obtained with both methods showed a positive linear association, being moderate for pO₂ (r=0.405) and very high for all others (r>0.86). Levels were concordant (r_c≥0.9), except for calcium (r_c=0.860), pH (r_c=0.853), pCO₂ (r_c=0.843) and pO₂ (r_c=0.336) and equivalent for all determinations except pCO₂ and pO₂, where clinically significant differences were found in more than 9% of cases (21.2%, 95% CI 16.6% to 26.5% and 73.1%, 95% CI 67.4% to 78.1%).

Conclusions Blood collection methods using DVP and PVC can be used interchangeably for most routine laboratory tests.