Common Test-Ordering Errors
Part 4: Overordered Tests
JAK2 on Blood and Bone Marrow
Free Thyroxine index (FTI) and T3 Uptake
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Published: July 2010Print Record of Viewing
Dr. Dale reviews test-ordering errors commonly seen at Mayo Medical Laboratories. This is the fourth presentation in the series and addresses overordered tests including ordering molecular tests (eg, JAK2) on both blood and bone marrow specimens, and commonly overordered thyroid tests.
Presenter: Jane C. Dale, MD
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Welcome to Mayo Medical Laboratories' Hot Topics. These presentations provide short discussions of current topics and may be helpful to you in your practice.
Our presenter for this 8-part series is Dr. Jane Dale, a consultant in the Department of Laboratory Medicine and Pathology at Mayo Clinic. Dr. Dale reviews test-ordering errors commonly seen at Mayo Medical Laboratories. This is the fourth presentation in the series and addresses overordered tests including ordering molecular tests (eg, JAK2) on both blood and bone marrow specimens, and commonly overordered thyroid tests.
Testing at Mayo Medical Laboratories
Here at Mayo Medical Laboratories we receive over 800,000 test orders each month. While by our estimation most of the orders we receive are appropriate orders, we do experience some recurring test-ordering problems. In this presentation, I will describe the most common problems that we see. I will also describe our efforts to decrease these problems and recommend ways that you too can help reduce these errors. Together, by decreasing inappropriate test orders, we can improve patient care and reduce costs.
Common Mayo Medical Laboratories Test-Ordering Errors
In this 8-part series, I will address 2 types of errors— misordered tests and overordered tests.
In parts 1 through 3 we discussed the most commonly misordered tests, that is, tests ordered for the wrong purpose. With this presentation, we begin our discussion of overordered tests, those that we believe are ordered too frequently, including a few that we do not recommend doing at all. This is a list of the most frequently overordered tests that we receive at Mayo Medical Laboratories.
In this 4th part of the series, we will address the first 2 bullet items— testing multiple specimen types for the same genetic abnormality and thyroid function testing. My first example is requests for the same molecular test on both blood and bone marrow specimens on the same patient during the same episode of care. Since the bone marrow is the source of peripheral blood cells, when looking for a genetic abnormality in hematopoietic cells, one only need test either the blood or the bone marrow, not both.
Let’s look at JAK2 as an example. The gene JAK2 encodes for a tyrosine kinase (called Janus kinase 2, also abbreviated JAK2). The JAK2 protein is involved in growth and differentiation of hematopoietic cells. A specific mutation in JAK2 (named V617F) is present in the hematopoietic cells of several chronic myeloproliferative disorders, most frequently in polycythemia vera, where it is present in approximately 90% of cases. If this mutation is present, it can be identified using either blood cells obtained by venipuncture or cells obtained by bone marrow aspiration—there is no need to order both tests. In fact, because of its high-negative predictive value (that is, its ability to rule-out polycythemia if not present), some have advocated use of the JAK2 test in peripheral blood as a means of eliminating the need for bone marrow biopsy in patients with clinical or lab features consistent with polycythemia vera. We are unclear why we receive dual orders as we assume that it is well known that a genetic marker present in blood cell precursors in bone marrow cells will also be present in those cells in the peripheral blood. We can only speculate that the tests are ordered at different times or by different physicians who are unaware of previous orders. Review of orders for duplicates prior to sending the request could resolve this problem.
Outdated Thyroid Function Tests
Next, I’d like to discuss 2 endocrine tests, free thyroxine index (FTI) and T3 uptake, which are 2 outdated tests that should no longer be ordered because we have better tests available. First, I’ll make a few general remarks about thyroid hormones to explain why the free thyroxine index was developed many years ago.
Thyroxine, called T4, is the main thyroid hormone. T4 circulates in 2 forms, protein bound (which accounts for over 99% of T4) and free. Free T4 accounts for <1% of circulating T4, but is the biologically active form of thyroxine. Both bound and free forms of thyroxine are measured by total T4 assays. While in most situations the total T4 test is a relatively reliable indicator of thyroxine status, when thyroid hormone-binding proteins are abnormal, total T4 results may be misleading. For example, increases in thyroxine-binding proteins can cause increased Total T4 levels despite normal free T4 levels and despite clinically normal thyroid function.
Assessing Thyroid Hormone Status in the Presence of Abnormal Binding Proteins
Hence, laboratory tests have been developed to compensate for the presence of abnormal types or quantities of thyroxine-binding proteins. The first of these was the free thyroxine index (FTI) and the T3 uptake test, which were developed in the 1950s. These tests came into use years before the free T4 assays we use today were technically possible, and they provided the first laboratory assessment of the amount of biologically active form of thyroxine.
Free Thyroxine Index (FTI)
The free thyroxine index is calculated from 2 measurements, total T4 and the T3 uptake (some of you may recall that the free thyroxine index was often referred to as a T7, referring to the T4 and T3 terms used in the calculation). I won’t belabor the details of the free thyroxine index test, except to say that the T3 uptake reflected the amount of thyroid-binding globulin (abbreviated TBG) that was bound by T4 and, thus, not available to bind radiolabeled T3 that was added to the test system. T3 uptake was, therefore, a way of adjusting total T4 concentrations for variations in thyroid-binding globulin. However, we now know that the free thyroxine index is unreliable in many situations. Many drugs and disease states, as well thyroid-binding globulin abnormalities, can alter the free thyroxine index, producing misleading results. The free thyroxine index and T3 uptake test are now considered outdated, replaced by better, direct measures of free T4.
Free T4 Assays
Modern direct measures of free T4, either by immunoassay or by dialysis, are much more accurate than the free thyroxine index. The immunoassay free T4 test is the preferred, routine test. It works well and is reliable in most settings, including when thyroid binding globulin abnormalities alter total T4 levels. The free T4 dialysis assay, is the gold standard reference method, and may be necessary in rare situations.
Free T4 Assay Summary
The free thyroxine index and T3 uptake offer no advantages over their newer replacement tests, but suffer from many more preanalytic and analytic variables. We believe that orders for free thyroxine index and T3 uptake should be questioned and, after educating the individual ordering those tests, replaced by free T4 tests. To learn more about this subject, please go to our Web site to access a Hot Topic on thyroid function testing. The Hot Topic was created by one of Mayo’s endocrinologists and provides a concise review of thyroid testing.
This concludes part 4 of this series on some of the tests that, in our experience as a reference laboratory, are commonly ordered inappropriately.