Lori Ann Lewis

Quality Control Procedures

Although much has been written about quality control procedures in densitometry, many of these articles have been concerned with data collection in clinical research rather than patient data collected as part of medical care. Quality control, while absolutely necessary in clinical research, is no less necessary in clinical practice. The original indications for bone mass measurements from the National Osteoporosis Foundation published in 1989 and the guidelines for the clinical applications of bone densitometry from the International Society for Clinical Densitometry published in 1996 called for strict quality control procedures at clinical sites performing densitometry [1, 2]. The Canadian Panel of the International Society for Clinical Densitometry published specific guidelines for quality control procedures in 2002 [3]. Such procedures are crucial to the generation of accurate and precise bone density data. When quality control is poor or absent, the bone density data may be incorrect. The interpretation made by the physician based on that incorrect information would be in error. The medical management of the patient may be adversely affected. The patient will also have been exposed to a small amount of radiation inappropriately and wasted time and money. In clinical trials, the results from hundreds or thousands of individuals are usually averaged and conclusions based on the average values. Small errors in machine performance are made insignificant by the averaging of so many results. In clinical practice, this luxury does not exist. Decisions are made based on one measurement from one patient, which means that strict quality control in clinical practice is even more important than in clinical trials. In spite of inherently superb accuracy and precision in today’s densitometers, alterations in the functioning of the machines can and will occur. Quality control procedures to detect these alterations in machine function should be utilized by every clinical site performing densitometry regardless of the frequency with which clinical measurements are performed.

Precision in Bone Densitometry

Precision is the attribute of a quantitative measurement technique such as bone densitometry that refers to the ability to reproduce the same numerical result in the setting of no real biological change when the test is repeatedly performed in an identical fashion. Like all quantitative tests in clinical medicine, no bone densitometry technique is perfectly reproducible. This is true even when the bone density test is performed in exact accordance with the manufacturer’ s recommendations every time. If the technologist does not position the patient and analyze the data in a consistent fashion, the technique becomes even less reproducible.

The Importance of Precision

Precision is the attribute of a quantitative measurement technique like bone densitometry that refers to the ability to reproduce the same numerical result in the setting of no real biologic change when the test is repeatedly performed in an identical fashion. Like all quantitative tests in clinical medicine, no bone densitometry technique is perfectly reproducible. This is true even when the bone density test is performed in exact accordance with the manufacturer’s recommendations every time. If the test is not consistently performed in accordance with the manufacturer’s recommendations, the technique becomes less reproducible.

Body Composition Analysis

Most full-size central DXA densitometers have software options to determine body composition from a total body bone density study. This application was first developed for dual photon absorptiometers, but the almost 1-h scan time made such measurements clinically impractical. In contrast, today’s DXA devices can perform total body scans in a matter of minutes. In spite of this dramatic improvement in speed, body composition assessment with DXA remains an underutilized application.

An Overview of Osteoporosis

Osteoporosis is not the only disease process in which bone densitometry is used in diagnosis and management. However, osteoporosis is perhaps the most important disease in which this technology is used, from the standpoint of the prevalence of the disease itself and the number of individuals referred for testing in the context of osteoporosis. It is not the responsibility of the technologist to discuss disease processes with patients referred for testing. In fact, some physicians would consider this intrusive and inappropriate. Nevertheless, the setting in which densitometry is usually performed and the interaction between the technologist and patient is conducive to patients asking the technologist questions about osteoporosis. In these circumstances, it would be inappropriate for a technologist to fail to respond within reason or appear to be uninformed and certainly counterproductive to the technologist-patient relationship. A knowledge of the disease process and the approved therapies for osteoporosis should be part of the densitometry technologist’s education. In any discussion with patients, however, it should also be emphasized that the patient’s physician is the final authority on the interpretation of bone density results and the need for prescription or nonprescription interventions to prevent or treat osteoporosis.

Interpretation of Bone Densitometry Data

The physician, not the technologist, is ultimately responsible for the interpretation of bone densitometry data. However, the ability of the technologist to perform his or her duties is enhanced by understanding what the physician will attempt to do with the data that they provide. Additionally, the circumstances in which bone densitometry is performed are quite different from the circumstances in which other diagnostic procedures are usually performed. For example, unlike having a chest X-ray in which the procedure is finished in a few seconds, the patient is often asked to sit or lie down for several minutes during a bone density study. The technologist is not physically separated from the patient by a protective barrier but usually seated only a few feet away. It is inevitable that the patient will ask questions about the test and the nature of the results. The technologist should be able to respond to these questions appropriately while ultimately deferring to the diagnostic judgment of the physician. It is also true that, occasionally, the technologist must make decisions independently regarding the choice of skeletal site to measure, the technical acceptability of the study, and the timing of return visits. An understanding of how the data are to be used is crucial to making these decisions.

Performing a DXA PA Lumbar Spine, Proximal Femur, or Forearm DXA Study

The most commonly studied regions of the skeleton remain to be the PA lumbar spine, proximal femur, and forearm in spite of the development of software applications that allow measurement of virtually any skeletal region. The manufacturers of DXA devices provide instructions for patient positioning for the different types of DXA studies that can be performed utilizing their device and software applications. While the manufacturer’s recommendations for positioning should always be given priority, an understanding of why certain aspects of positioning are recommended is useful. In particular, the technologist who understands the goals of positioning is better able to modify certain aspects of positioning when the patient’s anatomy demands it, without undermining the validity of the study. In reviewing the basic procedures and nuances of positioning for the three major scan types, the following discussion provides both the basic “how to” as well as the more detailed “why.”

Using Absolute Risk to Predict Fracture Risk in Clinical Practice

One of the most important uses of the bone density measurement is the prediction of the patient’s fracture risk. In the past, the statistical measure called relative risk (RR) was employed to convey the patient’s risk of fracture to the treating physician as well as to the patient. This resulted in statements such as “the patient’s risk of spine fracture is increased fourfold” or the “patient’s RR of spine fracture is 4.0.” Although this was the best that previously could be done, such statements actually conveyed little useful information. If a patient’s risk of spine fracture is increased fourfold, what does that really mean? The risk of fracture is fourfold greater than what? And while a fourfold increase in fracture risk sounded dire, it might not be. If the unstated baseline risk was extremely small, a fourfold increase in that extremely small baseline risk is still going to be very small. For this reason, the expression of a patient’s fracture risk in clinical practice as absolute risk (AR) is overwhelmingly preferred. The use of RR is considered obsolete and inappropriate. The utility of RR as a statistical expression of risk is unquestioned in clinical trials, but it has no role in clinical practice in the interpretation of DXA bone density data.

An Introduction to Conventions in Densitometry

In any discussion of bone densitometry, many terms and conventions are used that are unique to this field. In the chapters that follow, these terms and conventions will be used repeatedly. In an effort to facilitate the reading and comprehension of those chapters, a preliminary review of some of these unique aspects of bone densitometry is offered here.

Less Than Perfect Scan Images

Even though a technologist may follow positioning guidelines perfectly, the resulting study may be far from ideal because of artifacts or structural changes in the patient’s anatomy. Of the various skeletal regions of interest that may be studied with DXA, the lumbar spine is the region most commonly affected by artifact or structural change. The technologist obviously cannot correct structural changes in the patient’s skeleton, but it is imperative that they be recognized. Some structural changes or alterations will affect the measured BMD. Others may provide a possible explanation for the BMD value. Some artifacts are immediately recognizable while others are not. Any external artifacts should be removed if possible. If the artifact cannot be removed, the artifact should be identified and any possible effect on the measured BMD noted. Some explanations for structural changes or artifacts may be found in the patient’s history. The identification of others however is based purely on having seen it before. To that end, the DXA studies that follow reveal a variety of artifacts and structural changes encountered in clinical practice.