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Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109
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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