Here’s a summary of our June, 2012 Quick Question:
To perform the chromogenic anti-Xa heparin assay, how many separate reference curves do you maintain for unfractionated (UFH), low molecular weight (LMWH) heparin and fondaparinux?
a. One hybrid curve for UFH and LMWH, same curve (computed) for fonda: 7 (16%)
b. One hybrid curve for UFH and LMWH, a separate curve for fonda: 11 (25%)
c. One reference curve for each heparin: UFH, LMWH, and fonda: 22 (50%)
d. We don’t perform the chromogenic anti-Xa heparin assay: 4 (9%)
Most of us have chosen to stay with three curves rather than take advantage of the various manufacturers’ combined (hybrid) curve that may be used to measure either UFH or LMWH. I’m curious whether those who do not use the hybrid curve find it necessary to confirm the type of heparin being used before running the assay.
One of the reasons that I worked to develop the hybrid curve
One of the reasons that I worked to develop the hybrid curve cited in Blood Coagulation and Fibrinolysis 2005;16:173-6 and Lab Medicine 2005;36:1-3 was the problem of identifying the type of heparin being administered. The addition of extra calibrators for LMWH and UFH was a moot point since we already had to maintain separate calibration curves for several heparins and heparinoids we were being asked to monitor. By maintaining only one calibration curve and just running the appropriate controls we took away the guesswork for the coagulation laboratory.
Several vendors now provide hybrid curve calibrators ready-made in single vials. There is usually no cross contamination of different heparins or heparinoids used to anticoagulate subjects. Therefore the issue of the different ranges for each type of heparin should not pose a problem. The laboratory can then make the report of the concentration of heparin to the service monitoring the patient without having to bear the responsibility to identify the meds before testing. The companies that currently have the hybrid curve calibrators are Aniara, Stago, IL and Biophen. So far we have been able to use the hybrid curve to monitor UFH, enoxaparin, dalteparin and tinzaparin. Nordraparin has also been identified as being able to be monitored with the hybrid curve but it is not currently being used the the USA. I have seen mixed success monitoring fondaparinux using a hybrid curve so I still believe that a separate calibrator is necessary to adequately monitor that medication.
Dave McGlasson, 59th Clinical Research Division, Lackland AFB, TX
Hi George and fellow readers. I’ll chime in at this point i
Hi George and fellow readers. I’ll chime in at this point in the discussion.
I can see the utility of using a hybrid curve in the following settings:
1. the hospital core lab in which the anti-Xa has replaced the PTT for monitoring unfractionated heparin AND there is sufficient volume to support the additional expense of adding LMWH calibrators.
2. a national reference laboratory that has sufficient volume as above.
A significant number of your readers probably work in regional or metropolitan reference labs and, my assumption is, do not have the volume to support the additional cost of the hybrid curve.
To directly answer your question of “is it necessary to confirm the type of heparin being used before running the assay?” It is imperative that the type of heparin is confirmed prior to testing. The reason for this is very simple. The UFH calibration curve is significantly lower (0-0.6UI/ml) than the LMWH calibration curve (0-1.8UI/ml). Testing a LMWH sample on an UFH calibration curve would far exceed the linearity and reportable range of the curve.
St. Louis University Hospital Coagulation Reference Lab