Prolonged prothrombin time and activated partial thromboplastin time due to underfilled specimen tubes with 109 mmol/L (3.2%) citrate anticoagulant.
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.
Reneke
J
J
Department of Laboratory Medicine, University of California San Francisco School of Medicine, San Francisco General Hospital, 94110, USA.
Etzell
J
J
Leslie
S
S
Ng
V L
VL
Gottfried
E L
EL
D000328:Adult; D000925:Anticoagulants; D001800:Blood Specimen Collection / Q000379:methods*; D019343:Citric Acid; D005189:False Positive Reactions; D005260:Female; D006801:Humans; D007202:Indicators and Reagents; D008297:Male; D008875:Middle Aged; D010314:Partial Thromboplastin Time; D011517:Prothrombin Time; D012016:Reference Values; D013925:Thromboplastin; D014859:Warfarin / Q000627:therapeutic use
D000925:Anticoagulants; D007202:Indicators and Reagents; D019343:Citric Acid; D014859:Warfarin; D013925:Thromboplastin