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HelpTest Code LAB4257 Anti-Xa Assay, UFH
Specimen Type
Blue top tube (buffered 3.2% liquid sodium citrate) filled to the indicator with whole blood or Sodium Citrate Plasma removed from cells.
Specimen Volume
3mL Blue top (3.2% Sodium Citrate)
5mL Blue top (3.2% Sodium Citrate)
1mL Blue top (3.2% Sodium Citrate)
500uL (minimum) Sodium Citrate Plasma spun and removed from cells.
Turnaround Time
STAT: 1 hour
Routine: 4 hours
Test Schedule
Daily
Sample Stability
Room temp: 1 hour unspun, 2 hours spun. Platelet poor plasma is required.
Frozen samples are stable for 2 weeks at -20C or 12 months at -70C.
Method
Anti-Xa Chromogenic
Reference Ranges
Anti-Xa, UFH: ≤0.10 IU/mL
Critical Range: >1.0m IU/mL
Synonyms
Unfractionated Heparin, Anti-Xa; Heparin
CPT Codes
85520
Collection Instructions
Fill a Blue Top Container to the Indicator mark on the tube. Overfilled and underfilled tubes are not acceptable.
Specimen Processing
Samples should be centrifuged and run within one hour. If time interval between drawing and testing exceeds 1 hour, process sample to create platelet poor plasma (centrifuge specimen, separate plasma, centrifuge again, separate into plastic tube) and freeze at -20C. Platelet poor plasma is stable refrigertaed for 2 hours, Frozen (-20 C) for 2 weeks, or Frozen (-70 C) for 12 months.
Rejection Criteria
Severely hemolyzed specimens, clotted samples. Overfilled and underfilled tubes are not acceptable
Clinical Information
The anti-Xa test is used to quantitatively determine the plasma levels of unfractionated heparin (UFH) and low molecular weight heparin (LMWH) by measuring the anti-Xa activity on antithrombin in a competitive system using a synthetic chromogenic substrate. UFH and LMWH are used for prevention and treatment of thromboembolic diseases. The quantitative determination of these levels in plasma are useful for monitoring treatment efficacy.
The normal function of a molecule of factor Xa, as soon as it appears in plasma, is to cleave its natural substrate, prothrombin, to generate thrombin, the enzyme responsible for the fibrin clot formation. In the presence of heparin, competition occurs between this mechanism and the inhibitory mechanism exerted by the heparin-antithrombin III complex, made up of heparin and antithrombin (AT) from the patient. This inhibition is largely responsible for the action of heparin.
As soon as factor Xa is added to the plasma-substrate mixture, two reactions occur simultaneously: hydrolysis of the substrate by factor Xa and inhibition of factor Xa by the heparin-antithrombin III complex. After the necessary period for the competitive reaction to reach equilibrium, the quantity of para-nitroaniline (pNA) released is inversely proportional to the concentration of heparin present in the plasma.