Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant
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Published on: March 5, 2020
1Department of Laboratory Medicine, University of California San Francisco School of Medicine, San Francisco General Hospital, 94110, USA.
Insights
Specimen tube underfilling with 3.2% buffered citrate can falsely elevate prothrombin time (PT) and activated partial thromboplastin time (APTT). Fill tubes to at least 90% capacity for accurate coagulation test results.
Area of Science:
- Clinical Pathology
- Hematology
- Coagulation Diagnostics
Background:
- Underfilling citrate anticoagulant tubes is a known cause of prolonged prothrombin time (PT) and activated partial thromboplastin time (APTT).
- The impact of underfilling with 109 mmol/L (3.2%) buffered citrate on coagulation test results requires further investigation, particularly for patients on warfarin therapy.
Purpose of the Study:
- To evaluate the effect of underfilling 109 mmol/L (3.2%) buffered citrate tubes on PT and APTT measurements.
- To determine the minimum fill volume required for accurate PT and APTT results in healthy subjects and warfarin-treated patients.
Main Methods:
- Blood samples from 12 healthy subjects and 30 warfarin patients were collected into tubes with varying fill volumes (52%-100%) of 109 mmol/L buffered citrate.
- Prothrombin time (PT) and activated partial thromboplastin time (APTT) were measured using thromboplastin reagents with different International Sensitivity Index (ISI) values (1.01 and 2.06).
Main Results:
- Accurate PT results were obtained with fill volumes of 65% or more in normal specimens.
- Therapeutic range PT results required fill volumes of 80% (ISI=2.06) or 90% (ISI=1.01).
- APTT was less tolerant to underfilling, with prolonged values observed in most specimens filled below 90% capacity.
Conclusions:
- Specimen tubes containing 109 mmol/L (3.2%) buffered citrate should be filled to at least 90% capacity to ensure accurate PT and APTT results.
- Underfilling can lead to falsely elevated PT and APTT values, potentially impacting clinical decisions.
- While some underfilled tubes may yield results within the reference range, adherence to the 90% fill volume is recommended for reliability.
Abstract:
Underfilling of specimen tubes containing 129 mmol/L (3.8%) buffered citrate prolongs prothrombin time (PT) and activated partial thromboplastin time (APTT) values. We studied this phenomenon by using 109 mmol/L (3.2%) buffered citrate as the anticoagulant, anticipating some increase in tolerance to underfilling. Venous blood drawn from 12 healthy subjects and 30 patients receiving long-term oral warfarin therapy was mixed with 109 mmol/L buffered citrate solution in proportions equivalent to filling the collection tubes from 52% to 100% of capacity. Accurate PT values were obtained from normal specimens if the tubes were filled to 65% or more of capacity. Accurate PT results in the therapeutic range were obtained only with filling to 80% or more of capacity (using a "moderately sensitive" thromboplastin reagent, International Sensitivity Index [ISI] = 2.06) or 90% or more of capacity (using a "highly sensitive" thromboplastin reagent, ISI = 1.01). In contrast, APTT was much less tolerant to underfilling, with prolonged values observed in most specimens filled to less than 90% of capacity. No false low values were observed. Specimen tubes should be filled to at least 90% of capacity to avoid falsely elevated PT or APTT results, but values within the reference range may be acceptable even from underfilled tubes.
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