Test Catalog

Hot Topic

Common Test-Ordering Errors
Part 1: Misordered Tests

1,25-Dihydroxyvitamin D
PTH-related Peptide
Uroporphyrinogen III Synthase

Click CC box for captions; full transcript is below.

Published: April 2010

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Dr. Dale reviews test-ordering errors commonly seen at Mayo Medical Laboratories. This is the first presentation in the 8-part series and addresses 3 misordered tests: vitamin D, PTH-related peptide, and uroporphyrinogen III synthase.

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 first presentation in the series and addresses 3 misordered tests: vitamin D, PTH-related peptide, and uroporphyrinogen III synthase.

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 Test-Ordering Errors

In this 8-part series, I will address 2 types of errors—misordered tests and overordered tests.

Misordered Tests

In the first part of this series I will discuss some misordered tests, that is, tests that are mistakenly ordered for the wrong purpose. This slide shows the most commonly misordered tests received by Mayo Medical Laboratories. Today, we will limit our discussion to vitamin D, parathyroid hormone, and porphyria enzyme testing.

Vitamin D Assessment

The test for 1,25-Dihydroxyvitamin D is our #1 misordered test. 1,25-Dihydroxyvitamin D is often inappropriately ordered when the test for 25-Hydroxyvitamin D is indicated.

First, a bit of background to put this situation in proper context.

Vitamin D

Vitamin D plays a key role in calcium homeostasis and is vital for strong bones. Deficiencies may result in osteoporosis, rickets, osteomalacia, and secondary hyperparathyroidism.

Vitamin D also likely has important, emerging roles in many other aspects of health including muscle tone, immune function, diabetes, hypertension, heart disease, infections, cancer, and mental status (including depression in the elderly). Because of diet and life-style factors in the United States, it is generally accepted that a significant part of our population (especially children, young adults, pregnant women, the elderly, and individuals with dark skin) are vitamin D deficient. A small study of ICU patients reported in the New England Journal of Medicine found that over half of those patients were vitamin D deficient.

The CDC periodically assesses nutritional status in the United States via their National Health and Nutrition Examination Surveys (abbreviated NHANES). The latest NHANES data showed that over three-fourths of Americans did not have adequate vitamin D stores. Although more research is needed, we do know that physicians are increasingly interested in monitoring patients’ vitamin D status. This is reflected in our monthly test volumes for 25-Hydroxyvitamin D, the test that reflects vitamin D stores. However, we are also experiencing significant increases in orders for 1,25-Dihydroxyvitamin D, a form of vitamin D that is deficient in limited clinical situations.

Vitamin D Assessment

We believe that the analyte 1,25-Dihydroxyvitamin D is frequently mistakenly ordered to assess vitamin D stores and vitamin D sufficiency. Why is that?

Sources of Vitamin D

Vitamin D is obtained either in the diet or is derived intrinsically, a process involving sun exposure.

Vitamin D Pathway

I’d like to briefly review the vitamin D pathway because I think that helps explain the confusion about what test to order. Sun exposure, converts 7-dihydrocholesterol to vitamin D in the skin. Vitamin D is then converted to 25-Hydroxyvitamin D in the liver, and is stored in the tissues. 25-Hydroxyvitamin D reflects vitamin D derived from both dietary sources and vitamin D synthesized in the skin. A small fraction of 25-Hydroxyvitamin D is converted to the active metabolite 1,25-dihydroxy vitamin D, mainly by the kidneys, in a process regulated by parathyroid hormone.

Sunlight Exposure

Sun-exposure is an important variable and explains the lower levels of vitamin D seen in individuals with dark skin, upon which sunlight has decreased effect, in the elderly and other populations confined indoors, and in the seasonal variations that are seen in some areas of the country that have greatly reduced sunlight in the winter months.

Vitamin D Pathway

One reason that 1,25-Dihydroxyvitamin Ds are misordered may be that the individuals ordering vitamin D tests remember this pathway and assume that the end product, the biologically active form of vitamin D (1,25-dihydroxy vitamin D), is what you want to measure. But, that is not the case.

In most situations, when screening or monitoring for vitamin D deficiency or excess, it is the body stores that are the concern, and the correct test for that use is 25-Hydroxyvitamin D. 1,25-Dihydroxyvitamin D is an appropriate test, in certain situations. Recalling that the kidney is the site of synthesis of the active hormone, 1,25-Dihydroxyvitamin D testing plays a legitimate role in patients with renal failure. It is also a second-order test in the assessment of vitamin D status, especially for patients with uncommon disorders such as those with hypercalcemia and suppressed PTH, and patients with vitamin D-dependent rickets due to hereditary deficiency of renal 1-alpha hydroxylase or due to end-organ resistance to 1,25-dihydroxyvitamin D. We believe these indications together make up only about 20% of the 1,25-dihydroxyvitamin D orders we receive, with the remaining being ordered incorrectly.

Useful For

So, what have we done to mitigate the problem of inappropriate 1,25-Dihydroxyvitamin D orders? As for all our tests, we have defined the testing indications in our electronic and paper test ordering catalogs in the “Useful For” field. Comparing the “Useful For statements you see that 25-Hydroxyvitamin D is indicated for the “Diagnosis of vitamin D deficiency,” whereas 1,25-Dihydroxyvitamin D is indicated as a second-order test in the assessment of vitamin D status, especially in patients with renal disease. We have also produced a Hot Topic on vitamin D testing, which is accessible on our website, MayoMedicalLaboratories.com. I encourage you to check out this video.

Required Specimen Volumes

The test for 1,25-Dihydroxyvitamin D also is one of our most common “quantity not sufficient” (QNS) problems. We require 1.5 mL for 1,25-Dihydroxyvitamin D, while the preferred screening test (25-Hydroxyvitamin D), requires only 0.5 mL. While 1.5 mL may seem excessive, 1,25-Dihydroxyvitamin D concentrations are much lower than 25-Hydroxyvitamin D concentrations (1,25-Dihydroxyvitamin D is present at pg/mL levels, whereas 25-Hydroxyvitamin D is present at ng/mL levels). Therefore, more sample is necessary to measure at that lower level.

Parathyroid Hormone (PTH)

Another commonly misordered endocrine test is PTH-related peptide. We believe PTH-related peptide, abbreviated PTH-RP, is often mistakenly checked when PTH is wanted. PTH, parathyroid hormone, is a routine, commonly indicated test for a variety of parathyroid gland and calcium metabolism disorders. For example, increases in PTH (as seen in primary hyperparathyroidism) result in increases in serum calcium. Conversely, in primary hypoparathyroidism, decreases in PTH result in decreases in serum calcium.

The analyte PTH-Related Peptide is indicated in only a small subset of patients, namely those with hypercalcemia associated with malignancy. In these patients, calcium levels are high, but PTH levels are suppressed. This occurs because the tumor secretes a humoral factor that mimics the action of PTH. This factor is called PTH-related peptide. To prevent unintended orders for PTH-Related Peptide, it is important for ordering individuals to carefully read the complete test name and not confuse PTH with PTH-RP. Prior to ordering PTH-Related Peptide, patients should already have undergone PTH testing. Checking for that previously performed PTH result may be another way of assuring that PTH-Related Peptide is, in fact, the test that the physician wants.

Porphyria Enzymes

The next misordered test I would like to discuss is red blood cell Uroporphyrinogen III Synthase—a test for 1 of the porphyrias. In our experience, Uroporphyrinogen III Synthase is mistakenly ordered on patients with possible acute intermittent porphyria. For which the correct test to order is red blood cell Porphobilinogen (PBG) Deaminase.

These 2 enzymes, uroporphyrinogen III synthase and porphobilinogen deaminase, are deficient in 2 very different porphyrias—1 is a very rare cutaneous porphyria that presents in infancy with blistering skin lesions (named congenital erythropoietic porphyria) and the other is an acute porphyria (called acute intermittent porphyria) that presents in the teen ages or older with abdominal pain, neuropathy, and psychiatric symptoms. With such different presentations, why are these 2 tests confused? Here I think the answer is pretty simple. An alternate name for porphobilinogen deaminase, the enzyme deficient in most cases of acute intermittent porphyria , is uroporphyrinogen I synthase). Hence, it may be that uroporphyrinogen I synthase (the enzyme affected in acute intermittent porphyria) gets confused with uroporphyrinogen III synthase, the enzyme deficient in congenital erythropoietic porphyria. Again, individuals placing orders must pay particular attention to the test name to avoid this error. And, because congenital erythropoietic porphyria diagnosed in infants, before sending in a specimen for congenital erythropoietic porphyria on an adult patient, it would be helpful to confirm the indication with the ordering physician. This is especially critical because acute intermittent porphyria can be a medical emergency and ordering the right test and getting the correct diagnosis in a timely fashion is very important.


This concludes Part 1 of this series on some of the tests that, in our experience as a reference laboratory, are commonly ordered inappropriately. While each of you may find a different set of tests that fall into this category, it is my hope that by periodically reviewing test appropriateness, we can all find ways to improve. Some improvements require education, whereas others require procedural or systematic solutions. Some solutions are best addressed by those ordering the tests, whereas others are best addressed by those performing them. We look forward to hearing from you about ways that we at Mayo Medical Laboratories can help reduce test-ordering errors, as well as ways that you have found to do the same, striving to always assure that the right test is ordered, on the right patient, at the right time.

The algorithms and Hot Topic presentations referenced by me are available at MayoMedicalLaboratories.com, where you will also find the other parts of this series. You can also access issues of the Communiqué and our Test Catalog. For your convenience, our customer service phone number is 800-533-1710.

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