Antithrombin (AT) levels are frequently decreased in critically ill patients. It has been suggested that reduced AT levels result in underestimation of plasma heparin levels. Due to this concern, anti-activated factor X (Xa) assays may add excess AT to the sample to compensate for the AT deficiency. We sought to determine whether this added step, the addition of excess AT, was clinically useful by assessing the concordance between two Xa assays in ICU patients with reduced AT levels receiving continuous unfractionated heparin (UH) infusions; the STA-Chrom® Heparin assay adds excess AT to the sample before assessing the heparin level, whereas the STA-Rotachrom® assay does not.
We conducted a prospective evaluation of forty-one consecutive ICU patients receiving continuous UH infusions. Blood samples drawn to monitor UH therapy had heparin levels measured (by the two different assays listed above), as well as AT activity levels. The concordance between the Xa assays in AT depleted patients was determined.
13 of the 41 patients were noted to have an AT level less than 60% of normal human plasma activity. A total of 30 pairs of Xa levels were determined from this population. In AT depleted patients, Xa levels measured by the STA-Chrom® and STA-Rotachrom® assays were similar (correlation between paired values was 0.87).
The concordance between Xa assays in AT depleted ICU patients is high. We found no benefit in an assay that adds excess AT to the sample prior to measuring plasma heparin levels in patients treated with UH.
The management of anticoagulation with UH, in critically ill patients deficient in AT, would not be altered by this Xa assay that added excess AT to plasma samples.
B.A. Markewitz, None.