Bone scans are frequently performed to diagnose bone and joint diseases. The scan is an imaging test that uses a radioactive tracer to reveal bone damage or diagnose a disease. Bone scans reveal the amount of radioisotopes or X-rays that are absorbed by the bone tissue. The difference in the amount of these substances absorbed by the normal and the affected or unhealthy tissues can provide a clues about the diagnosis.
A bone scan can help in the early detection of diseases by revealing the change in the absorption of radioisotopes by lesions that depend primarily on the rate of bone turnover.
Although bone scans are very sensitive and can usually provide accurate results, the findings may sometimes be wrong. There is a chance of misinterpreting the findings and the disease diagnosis may be affected. Here is a brief discussion about when bone scans can go wrong.
What are Bone Scans
Bone scans are the tests performed to measure bone density. These tests are also called bone densitometry tests. Dual-energy x-ray (DEXA) scans are commonly performed bone scans that use small doses of ionizing radiation to produce images of the internal organs and tissues of the body including the bones. 
A bone scan usually involves the hip bone and lower spine to measure bone loss. It is a commonly used method for the diagnosis of osteoporosis or to assess the risk of developing osteoporotic fractures.
According to a researcher, Myung-Hee Sohn, “A bone scan is extremely sensitive for detecting bony abnormalities. However, abnormalities that appear on the bone scans may not always represent disease. The normal scan appearances may be affected not only by skeletal physiology and anatomy but also by a variety of technical factors, which can influence image quality. Many normal variants and artifacts may appear on bone scan. They could simulate a pathologic process and could mislead into the wrong diagnostic interpretation. Therefore, their recognition is necessary to avoid misdiagnosis.”  
Let’s have a look at the common mistakes that may occur in bone scans performed for assessing bone mineral density (BMD).
What are the causes of errors in the results of bone scans?
It’s common to perform regular bone scans for a BMD measurement to monitor a person’s bone density and to assess any effects of therapeutic intervention.
A repeat of the bone scan is recommended when a physician needs to distinguish between true change in bone mineral density and random fluctuations that may occur during the test.
The variability in the results of bone scans may occur due to multiple causes including device variations, patients’ movements, technician variability, change in the area of focus, and so on.
Let us learn more about DEXA scans and various reasons that can influence the results of these tests.
The bones involved
The accuracy of DEXA scans depends on the variable composition of the bones and soft tissues. The differences in the composition of the soft tissue in the path of x-ray beams passing through the bones compared with that of the adjacent soft tissues may significantly influence BMD measurement.
The variation is higher when the test is performed in the bones of the femoral area, neck, hips, and spine than in the bones of the forearm. 
Errors in interpretation
Errors in interpretation can occur due to the incorrect labeling of vertebral bodies and comparing T-scores instead of the bone mineral density. Interpretation errors can also result from comparing the normal bone density of pre-menopausal women to that of post-menopausal women. 
The precision error can be expressed as the CV or the coefficient of variation that is calculated as the ratio of the standard deviation to the mean of all measurements.
Several statistics are used to express precision error including SDD (The smallest detectable difference) and LSC (The least significant change). The SDD indicates a cut-off, which can be measured for a particular individual and is considered more useful than a CV.
Precision error is common for most scans including bone scans performed to measure bone densitometry. However, this error can be avoided by undergoing the test under the observation of an expert radiologist who can account for the precision error while interpreting the findings.
Incorrect BMD acquisition
Incorrect BMD acquisition and reporting might generate inappropriate therapeutic and clinical decisions. This could have an adverse effect on the patient’s health. These errors may result from the improper installation and maintenance of the equipment.
According to a research study published in the Current Radiology Reports, “BMD testing, interpretation and reporting are erroneously considered to be simple; instead, all these steps require training and experience for technologists and physicians.”  
It has been reported that 45% of technologists and 71% of clinicians have incorrect densitometric acquisition and interpretation nearly once a month. These errors usually occur due to the inability to adhere to the recommendations by the manufacturer for quality control and maintenance of the device. 
Additionally, inadequate training and education in bone densitometry and lack of knowledge of the latest guidelines may also contribute to errors in the results of bone scans. 
Change in circumstances
Ideally, the same technologist should perform DEXA scans for a particular patient on the same device and under similar circumstances to avoid errors in results. Undergoing these tests at different clinics each time or by a different technologist may show variations in the results that may or may not be due to the actual decline or improvement in the patient’s bone density. These factors must be taken into consideration while interpreting the results.
DEXA scan is recognized as a reference method for the measurement of bone mineral density with good precision and accuracy. DEXA scans are established as the best densitometric methods for assessing BMD, especially in postmenopausal women.
These scans allow for the accurate diagnosis of osteoporosis and provide an estimation of the risk of fractures.
However, variables during DEXA scans may lead to an incorrect assessment of the patient’s BMD. It is advisable to visit an experienced radiologist who knows the variations in technical factors and can interpret bone scans correctly.