title: To mix or not to mix venous blood samples collected in vacuum tubes?
abstract: BACKGROUND
  There are recommendations to mix venous blood samples by inverting the tubes immediately after venipuncture. Though mixing allows efficient anticoagulation in plasma tubes and fast initiation of coagulation in serum tubes, the effect on laboratory analyses and risk of haemolysis has not been thoroughly evaluated.
  
  
  METHODS
  Venous blood samples were collected by venipuncture in vacuum tubes from 50 patients (10 or 20 patients in each group). Four types of tubes and 18 parameters used in routine clinical chemistry were evaluated. For each patient and tube, three types of mixing strategies were used: instant mixing, no mixing and 5 min of rest followed by mixing.
  
  
  RESULTS
  Most analyses did not differ significantly in samples admitted to different mixing strategies. Plasma lactate dehydrogenase and haemolysis index showed a small but significant increase in samples omitted to instant mixing compared to samples without mixing. However, in one out of twenty non-mixed samples, activated partial thromboplastin time was seriously affected.
  
  
  CONCLUSIONS
  These results indicate that mixing blood samples after venipuncture is not mandatory for all types of tubes. Instant mixing may introduce interference for those analyses susceptible to haemolysis. However, tubes with liquid-based citrate buffer for coagulation testing should be mixed to avoid clotting.
authors: Division of Clinical Chemistry, Department of Clinical and Experimental Medicine, Faculty of Health Sciences, Linköping University, County Council of Östergötland, Linköping, Sweden.
authors: Anna
authors: A
authors: Parenmark
authors: Eva
authors: E
authors: Landberg
mesh_terms: D019963:Clinical Chemistry Tests; D006454:Hemoglobins / Q000032:analysis; D006461:Hemolysis; D006801:Humans; D010314:Partial Thromboplastin Time; D018962:Phlebotomy / Q000379:methods*
chemical_list: D006454:Hemoglobins