Prothrombin time determination. The lack of need for a discard tube and 24-hour stability

Consensus instability equations for routine coagulation tests

OBJECTIVES

The stability of plasma samples for basic coagulation tests, prothrombin time (PT) and activated partial thromboplastin time (aPTT), has been widely studied. Recently, the Clinical and Laboratory Standards Institute (CLSI) updated its recommendations, extending the acceptable time frame for aPTT. These guidelines are based on experimental studies, which define limits according to different maximum permissible error (MPE) criteria. This study compiles raw data from 43 studies published over the last 30 years to develop a consensus instability equation that describes degradation independently of specific study parameters.

METHODS

A critical literature review was performed by collecting studies that included experimental stability data for PT, aPTT and the main procoagulant factors. The raw data of percentage deviation (PD%), time, and seven classification variables related to sample collection and handling were analysed. A regression model through the origin was applied to derive global instability equations and to assess influencing variables.

RESULTS

In frozen samples, PT and aPTT showed similar stability, with an average prolongation of 0.8 % per month. In non-frozen samples, tube handling affected stability more than storage temperature. The consensus equation for PT showed a linear average deterioration of 2.9 % per day, but model strength was limited. For aPTT, the consensus equation fitted better to a logarithmic decay model and predicted prolongations of 6.1 and 10 % at 6 and 24 h, respectively.

CONCLUSIONS

The consensus instability equations obtained in this review provide a robust model for assessing coagulation tests stability, aligning with expert recommendations. These equations improve the understanding of sample degradation and systematic error quantification.

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.

External quality assessment of prothrombin time: The split‐sample model compared with external quality assessment with commercial control material

OBJECTIVE

CoaguChek S is a point-of-care, whole-blood, prothrombin time monitor. The purpose of this study was to compare two different methods for external quality assessments of CoaguChek S.

MATERIAL AND METHODS

In the traditional external quality assessment scheme, commercial control material was sent to office laboratories and the results were compared with a method-specific target value. In the alternative external quality assessment (the split-sample survey) patient samples were analyzed on CoaguChek S at office laboratories, and venous blood samples from the same patients were analyzed at a hospital laboratory using an assigned comparison method. To obtain comparable performance criteria for the two methods, the limits for "good", "acceptable" and "poor" performance evaluation in the split-sample survey had to be expanded because of uncertainties in preanalytical factors and the comparison method.

RESULTS

In the traditional external quality assessment the total imprecision (between-office and within-office) was 8.0% at the low level (1.6 INR (International Normalized Ratio)) and 10.5% at the therapeutic level (3.4 INR). In the split-sample survey the total imprecision was 12.3% at the low level (2.1 INR) and 10.7 % at the high level (3.0 INR). Seventy-five percent of the participating office laboratories were characterized as "good" with the traditional external quality assessments, whereas the corresponding number was 73% using the split-sample model.

CONCLUSIONS

Available commercial control material for CoaguChek S is different from patient samples. This study demonstrates that split-sample survey is achievable, and is an acceptable alternative to traditional external quality assessment for point-of-care prothrombin time monitors where appropriate control material is difficult to obtain.

Non-frozen transports of whole blood samples do not cause relevant bias for global coagulation tests in clinical trials evaluating the drug safety

Sample shipments with dry ice have a large economic impact on clinical research. Therefore, the bias caused for global coagulation tests by non-frozen transports of whole blood instead of frozen plasma was investigated experimentally and by a meta-analysis of 6-year central laboratory data. In the experiment, aliquots from 14 healthy volunteers were kept as whole blood at 20+/-2 degrees C and as frozen plasma until an analysis of prothrombin time (PT), activated partial thromboplastin time (aPTT), thrombin time (TT), and antithrombin III (ATIII) at day 0, 1, 2, and 3 from collection. Within these 3 days only PT and aPTT demonstrated any changes: in blood samples kept at 20+/-2 degrees C these amounted about 10% for both. In frozen plasma, aPTT did not change whereas PT increased by 14%. In a meta-analysis of central laboratory data, PT and aPTT results were grouped across various phase II-IV trials by the type of sample transfer, either as frozen plasma on dry ice or non-frozen as whole blood. For the latter the mean difference to a reference group of phase I trials with same-day analysis was in line with the amount of bias found in the experiment (aPTT, 34.6+/-6.0 vs. 31.6+/-3.5 s; PT, 87.7+/-13.3 vs. 97.3+/-7.9%). The consistent bias resulted in shifted, but still normal distribution curves with a total rate of clinically relevant outliers of about 1.9% for aPTT and 2.4% for PT. Biases thus appear irrelevant for a common safety evaluation within clinical trials. Non-frozen whole blood transports for the measurement of global coagulation tests appear justified for this purpose, if protocols do not require frozen shipments for other reasons. However, transit time must not exceed 2 days and pre-analytical conditions should be consistent within the same trial.

Preanalytical Variables and Off-Site Blood Collection: Influences on the Results of the Prothrombin Time/International Normalized Ratio Test and Implications for Monitoring of Oral Anticoagulant Therapy

Background: The quality of oral anticoagulant therapy management with coumarin derivatives requires reliable results for the prothrombin time/International Normalized Ratio (PT/INR). We assessed the effect on PT/INR of preanalytical variables, including ones related to off-site blood collection and transportation to a laboratory.

Methods: Four laboratories with different combinations of blood collection systems, thromboplastin reagents, and coagulation meters participated. The simulated preanalytical variables included time between blood collection and PT/INR determinations on samples stored at room temperature, at 4–6 °C, and at 37 °C; mechanical agitation at room temperature, at 4–6 °C, and at 37 °C; time between centrifugation and PT/INR determination; and times and temperatures of centrifugation. For variables that affected results, the effect of the variable was classified as moderate when &lt;25% of samples showed a change &gt;10% or as large if &gt;25% of samples showed such a change.

Results: During the first 6 h after blood collection, INR changed by &gt;10% in &lt;25% of samples (moderate effect) when blood samples were stored at room temperature, 4–6 °C, or 37 °C with or without mechanical agitation and independent of the time of centrifugation after blood collection. With one combination of materials and preanalytical conditions, a 24-h delay at room temperature or 4–6 °C had a large effect, i.e., changes &gt;10% in &gt;25% of samples. In all laboratories, a 24-h delay at 37 °C or with mechanical agitation had a large effect. We observed no clinically or statistically relevant INR differences among studied centrifugation conditions (centrifugation temperature, 20 °C or no temperature control; centrifugation time, 5 or 10 min).

Conclusions: We recommend a maximum of 6 h between blood collection and PT/INR determination. The impact of a 24-h delay should be investigated for each combination of materials and conditions.

Reliability of delayed prothrombin time INR determinations in a central laboratory using off-site blood sampling

A major concern of centralized anticoagulant measurements with off-site sampling is the reliability of international normalized ratio (INR) determinations on blood that may have been taken from the patient hours before the analysis. We compared INR differences in the blood of patients receiving oral anticoagulants after 24 h storage in four conditions: centrifuged at room temperature, centrifuged at 4 degrees C, uncentrifuged at room temperature and uncentrifuged at 4 degrees C. The INR of centrifuged and uncentrifuged blood left at room temperature for 24 h consistently increased by 6% and, after adjustment, there were no misclassifications in the assessment of the adequacy of anticoagulant treatment. Inconsistent changes were noted in tests of refrigerated centrifuged blood. We conclude that storage of blood at room temperature for 24 h results in a consistent prolongation of the prothrombin time, which after correction can reliably be used to adjust the dose of oral anticoagulants.

Stability of prothrombin time and activated partial thromboplastin time tests under different storage conditions

Prothrombin time (PT) and activated partial thromboplastin time (aPTT) are common laboratory tests that are useful in the diagnosis of coagulation disorders and monitoring anticoagulant therapy. Recent expansions in the outreach laboratory services at our institution prompted us to investigate the shipping limitations for some tests, including PT and aPTT. Although we followed NCCLS guidelines for the collection of blood specimens, we observed falsely elevated PT and aPTT values due to the different storage conditions. The objective of this study is to determine the effect of conditions and duration of storage on PT and aPTT tests using plasma and whole blood samples, respectively. For this study, 36 plasma samples with normal and prolonged PT and aPTT were exposed to different storage conditions. Blood was centrifuged immediately and plasma was stored at room temperature (RT), refrigerated at 4 degrees C, or frozen at -20 degrees C. The samples were analyzed at 0 h and repeated at 6, 12 and 24 h under various conditions. Although statistically significant differences were observed for plasma samples for normal PT tests after 12 h at refrigerated and frozen storage conditions, the differences would not change the clinical interpretation of the results. On the other hand, samples stored refrigerated or at RT showed significant differences for aPTT at 24 h. These differences would change clinical interpretation, especially for samples with normal or near normal aPTT times. Interestingly, aPTT was significantly higher for samples stored frozen when compared to refrigerated and RT conditions at 6 h. Similar patterns were also observed on ten whole blood samples with normal PT and aPTT values. In conclusion, either plasma or whole blood samples can be accepted for PT testing up to 24 h and for aPTT testing up to 12 h only, when transported either at RT or at 4 degrees C.