Prothrombin time: one tube or two

Order of draw practices in venous blood sampling at clinical biochemistry departments in the Danish health care system

BACKGROUND

Deviation in blood collection procedures is a central source of preanalytical variation affecting overall analytical and diagnostic precision. The order of draw of venous sampling is suspected to affect analytical results, in particular for coagulation analysis. Here we compare the procedures in venous blood sampling among clinical biochemistry departments to assess the uniformity of order of blood draw and adherence to international guidelines in the Danish health care system.

METHODS

We collected venous order of draw procedures from 49 clinical biochemistry departments at 22 public hospitals in Denmark. Procedures were compared to the international guidelines fromthe Clinical Laboratory Standards Institute (CLSI) and World Health Organization (WHO), and assessed in relation to department ISO 15189:2012 accreditation.

RESULTS

We observed seven different order of draw procedures related to citrate, serum, heparin, and EDTA tubes, and the use of discard tubes in relation to coagulation assays. 31 departments (63.3%) were found to adhere to CLSI and WHO guidelines. A majority of departments instructs the use of discard tubes before collection for coagulation assays in citrate tubes (44 departments; 89.8%). The citrate tube was the first sample tube to be drawn for most departments (35 departments; 75.5%); and the preferred order of non-citrate tubes was serum-heparin-EDTA (36 departments; 73.5%). Adherence to the CLSI and WHO guidelines was not associated with department ISO 15189:2012 accreditation (p = .57).

CONCLUSIONS

Venous order of draw procedures is diverse at Danish clinical biochemistry departments and show moderate adherence to international guidelines.

Preanalytical investigations of phlebotomy: methodological aspects, pitfalls and recommendations

Phlebotomy is often addressed as a crucial process in the pre-analytical phase, in which a large part of laboratory errors take place, but to date there is not yet a consolidated methodological paradigm. Seeking literature, we found 36 suitable investigations issued between 1996 and 2016 (April) dealing with the investigation of pre-analytical factors related to phlebotomy. We found that the largest part of studies had a cohort of healthy volunteers (22/36) or outpatients (11/36), with the former group showing a significantly smaller median sample size (N = 20, IQR: 17.5-30 and N = 88, IQR: 54.5-220.5 respectively, P < 0.001). Moreover, the largest part investigated one pre-analytical factor (26/36) and regarded more than one laboratory test (29/36), and authors preferably used paired Student’s t-test (17/36) or Wilcoxon’s test (11/36), but calibration (i.e. sample size calculation for a detectable effect) was addressed only in one manuscript. The Bland-Altman plot was often the preferred method used to estimate bias (12/36), as well as the Passing-Bablok regression for agreement (8/36). However, often papers did assess neither bias (12/36) nor agreement (24/36). Clinical significance of bias was preferably assessed comparing to a database value (16/36), and it resulted uncorrelated with the size of the effect produced by the factor (P = 0.142). However, the median effect size (ES) resulted significantly larger if the associated factor was clinically significant instead of non-significant (ES = 1.140, IQR: 0.815-1.700 and ES = 0.349, IQR: 0.228-0.531 respectively, P < 0.001). On these evidences, we discussed some recommendations for improving methodological consistency, delivering reliable results, as well as ensuring accessibility to practical evidences.

Routine coagulation testing: do we need a discard tube?

When coagulation tests are performed, the recommended guideline is that a discard tube be used and the coagulation testing should be done only on the second tube. This guideline is however inconsistently enforced and most laboratories follow a single tube draw for routine coagulation testing. Few studies have however, challenged this guideline and have shown that comparable results can be obtained in both tubes when a two tube draw is used. This prospective study was done over a 3 months period in the hematology laboratory under the Clinical Hematology unit of a tertiary care teaching institution in North India. Fifty-six paired specimens were drawn from healthy volunteers following the prescribed "two tube draw" method. Prothrombin time (PT) and activated partial thromboplastin time (APTT) were performed within 1 h of sample collection on a fully automated photo-optical coagulation instrument (Ceveron-Alpha). Paired results for PT and APTT were compared using Bland-Altman plots for method comparison. There was good correlation between the PT, INR and APTT of the first and second tubes with bias of 0.09, -0.05 and 0.3 respectively). Bland-Altman plots showed acceptable agreement between the two values with 95 % confidence interval ranging from -0.62 to 0.79 for PT, -0.05 to 0.06 for INR and -3.9 to 4.6 for APTT. Our study has shown no significant difference between PT and APTT values for the first and second tubes. Hence the use of a discard tube is not required.

Discard first tube for coagulation testing

Tissue thromboplastin may contaminate the first tube sample due to the trauma of the venipuncture, and therefore, affect the accuracy of coagulation testing. This practice was stopped by Clinical and Laboratory Standards Institute after several studies. However, most of the studies have verified these conclusions and refuted the need for a discard tube when drawing samples for coagulation tests in healthy groups. The purpose of our study was to evaluate the clinical importance of discarding a tube for prothrombin time (PT) determinations on large samples with international normalized ratio (INR) values between and over targeted therapeutic range. Patients receiving oral anticoagulation therapy (OAT) managed by our cardiology service were selected for this study. Tube 1 was always treated as the discard tube. Tube 2 was allocated to be analyzed along with the tube 1 for coagulation tests. Individual values were grouped into four cohorts according to the INR range. The ranges were as follows: less than 2.0, 2.1-3.0, 3.1-4.5 and more. Three hundred and seventy-six samples were drawn for PT/INR and activated partial thromboplastin time testing. We found statistically significant differences between tube 1 and tube 2 (P < 0.05), and satisfactory correlation coefficients were obtained by linear regression analysis (0.86 or greater in all cases). This study consisted of a high number of samples. Our data suggest that drawing a discard tube is still necessary for coagulation testing. Consideration should be given to revising the international guidelines related to the necessity of a discard tube for repeated evaluation of coagulation tests especially receiving long-term OAT.

Collection of blood specimens by venipuncture for plasma-based coagulation assays: necessity of a discard tube

The Clinical and Laboratory Standards Institute (CLSI) recently abandoned its recommendation for drawing a discard tube when performing a prothrombin time (PT)/international normalized ratio (INR) or an activated partial thromboplastin time (APTT). Because there is currently no evidence that a discard tube is necessary for more specialized coagulation assays, we studied the need for a discard tube for some of these tests. Blood was obtained from 88 subjects in 2 subsequent citrate tubes. Platelet-free plasma was tested for PT, APTT, antithrombin, protein C, and factors II, V, VIII, IX, and X. Difference and bias between tubes were tested using the Wilcoxon signed rank test and Bland-Altman plots. For only APTT, antithrombin, and protein C was a small, statistically significant mean bias found (0.5 seconds; P = .001; -0.7%, P = .002; and -0.8%, P < .0001, respectively), but the bias of individual samples was not clinically relevant. This was also true for the other parameters tested. The recent CLSI recommendation that a discard tube is not necessary for PT/INR and APTT can be extended to include more specialized plasma-based coagulation assays as identified in this study.