CAP TODAY Pathology/Laboratory Medicine/Laboratory Management
Dr. Pruthi
“The half-life of the current generation of unmodified factor concentrates varies,” said Dr. Pruthi, who is also an associate professor, Mayo Clinic College of Medicine, and co-director of Mayo Clinic’s special coagulation laboratory. For factor VIII, the half-life is between eight and 12 hours, whereas the half-life for factor IX is between 18 and 20 hours. “Based on this, the typical practice is to infuse the unmodified factor VIII concentrate about three times weekly, and the factor IX concentrate is typically administered two times weekly.”
The target trough factor level is usually greater than one percent. “Typically, we like to target between one and five percent,” Dr. Pruthi said. “Targeting that level completely changes the frequency of bleeding that severe hemophilia patients experience.” However, there’s wide variability between patients in the half-life of these factors. To provide the most cost-effective therapy, individualized pharmacokinetic studies are usually performed so the dosing can be tailored to each patient.
He shared a slide illustrating the results of a typical pharmacokinetic study for an unmodified and a modified factor VIII concentrate. In this trial, the half-life of the standard factor VIII was compared with that of a modified factor VIII. The data illustrate the time it takes for the factor levels to decrease from a post-infusion level of about 100 percent to a trough of between one and three percent. For the unmodified concentrate, it takes about three days to reach that level; for the modified factor VIII concentrate, it takes about five days. It takes about four days for the standard factor IX concentrate to get down to between one and three percent, but with the modification of the factor IX molecule, the time to a trough level of one to three percent is extended to about 10 days.
“There is a wide inter-individual variability,” Dr. Pruthi reminded the audience. “One of the consequences of inaccurate measurements is that with under-dosing of the factor concentrate, you may increase the risk of bleeding. However, if you over-dose the factor concentrate, you may increase the risk of thrombosis. You certainly will be increasing the cost of care.”
He presented the case of a 20-year-old male with severe hemophilia A who was switched to a modified factor VIII concentrate, and whose pre-infusion baseline factor level was less than one percent (normal range, 55 to 200 percent). “We calculated the dose he would require to target a peak factor VIII level of approximately 60 percent,” Dr. Pruthi explained. “However, when we measured his post-infusion level, it was actually only measured at 30 percent using the one-stage assay. So when this happens, there are several questions one has to address.” For example: Were the sample collection and transportation done correctly? Was the assay the right assay for this concentrate? Did he receive the ordered dose?
“Once the preanalytic and analytic aspects of the assay have been investigated and the assay’s result is not felt to be erroneous, the typical next step is to increase the dose of the recombinant factor concentrate and recheck the pharmacokinetics,” he continued. “Now, if that result was inaccurate and we would be increasing the dose, then definitely we would be increasing the cost of care and potentially putting the patient at a higher risk of thrombosis.”
His team realized the reagents used for the one-stage assay performed on this plasma sample underestimated the true factor level by about 50 percent for this modified factor concentrate. In fact, the package insert for the modified concentrate recommends that the one-stage assay result be multiplied by a factor of two. In other words, the patient was on the right dosage—it was just that the one-stage assay result had to be multiplied by two. The result of a chromogenic factor VIII assay confirmed such.
In the second case, a 12-year-old male with severe hemophilia B was referred to Mayo Clinic’s hemophilia center for help switching to the new modified recombinant factor IX concentrate. His dosing was calculated to achieve a trough level of five percent, and indeed his pre-infusion factor IX was five percent. An hour post-infusion, he reached a peak level of 80 percent.
“So the patient was referred back to his local care provider with the advice that the pharmacokinetics should be rechecked at some point, and between going back to his provider and coming in for a recheck of his pharmacokinetics, the patient experienced no bleeding events since initiation of the prophylaxis,” Dr. Pruthi said.
But when the pharmacokinetics were rechecked at one point, the results—obtained in a local laboratory—demonstrated that his pre-infusion or trough level was less than one percent, while his post-infusion peak level was only 40 percent.
The patient’s primary care provider had planned to increase the dose of the modified factor IX concentrate but contacted the Mayo Clinic hemophilia center for advice. The center determined that the local laboratory was using an aPTT reagent based on a kaolin activator for the one-stage assay, and this kaolin activator was known to underestimate the true factor IX level for this particular product. A sample was mailed to the laboratory affiliated with the hemophilia center, which confirmed the results of the original pharmacokinetic study.
“So the underestimation of the true factor level has a significant consequence,” Dr. Pruthi stressed. “You may be increasing the dose of factor infusion, overdosing the patient, increasing the cost, and putting the patient at risk for thrombotic complications. Whereas if you overestimate the true factor level, the potential consequence is you would reduce the dosage of the factor concentrate and potentially increase the risk of bleeding.”
This is a complex situation with multiple potential solutions, some more practical than others. For example, each laboratory could have an individualized calibrator for each concentrate for which it will potentially perform assays. However, the information regarding which concentrate the patient is on might not be communicated to the laboratory. And maintaining assays with different calibrators poses special challenges to both low- and high-volume laboratories.
“What about chromogenic assays?” he said, referring to Dr. Tiefenbacher’s outline of the available factor VIII kits. “There are no currently FDA-approved factor IX kits. And so each lab would have to validate a kit as a laboratory-developed test, which poses unique regulatory challenges and is very time-consuming and expensive.” Finally, one could multiply the one-stage assay result by a correction factor, as one of the cases showed. “However, each hemophilia care provider would have to be aware of such recommendations to ensure that the correct correction factor is being applied.”
Dr. Pruthi concluded by stressing again that exclusively using one type of assay may lead to misclassification of non-severe hemophilia or even a missed diagnosis, and when monitoring factor concentrates, may lead to over- or underestimating factor levels. “Hemophilia care providers should be made aware of these assay-related issues,” he said, so as to avoid risking incorrect dosage adjustments of the factor concentrates.
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Anne Ford is a writer in Evanston, Ill. See the March 2017 issue for the guidance of Dorothy M. Adcock, MD, on the initial evaluation of non-severe hemophilia A.