SAN FRANCISCO — Plain x-rays are insufficiently quantitative to assess a child's bone mineral density, and dual-energy x-ray absorptiometry is usually the best choice, said Dr. Laura K. Bachrach at a meeting of the Society for Pediatric Dermatology.
Dual-energy x-ray absorptiometry (DXA) is fast, precise, safe, and readily available. But interpreting the results can present some challenges to the nonspecialist, said Dr. Bachrach of Stanford (Calif.) University.
“There's often a very inadequate response from the DXA center or nuclear medicine department or the radiologist who prepares the report,” Dr. Bachrach said. “[Often they] produce some numbers and throw up their hands and say 'Here, you ordered it, you interpret it.'”
The results will be in units of bone mineral content (BMC) in grams or bone mineral density (BMD) in g/cm
Some clinicians make the mistake of relying on the standard deviation T score, but this is particularly inappropriate in children. A T score compares a patient's bone mineral density to that of a healthy young adult. Children with normal bone mineral density will often appear to have abnormally low BMD by that standard.
The z score, on the other hand, compares the child's result to children of the same age and sex.
It's also important to pay attention to both the make and the model number of the DXA machine. Normative data for one machine should not be used to analyze results from another.
But even a z score is not enough, Dr. Bachrach said. “Usually the children you're ordering the DXA scan on have a chronic illness, and often these illnesses have affected their growth, maturation, and have really slowed things down. These can affect the results. And this is where you may want to get on the phone and talk to your friendly bone-density colleague. There's definitely a tendency to overdiagnose low bone mass, most commonly because of the T score error, but also potentially if you don't take bone size into account.”
Dr. Bachrach said that patients frequently call, saying, “My doctor says my child's losing bone.” But it's simply not possible to determine that with a single DXA scan. More likely, the child is not losing bone, but he or she is failing to accrue bone.
Additionally, because it's unknown what a given child's fracture threshold is, it's impossible to diagnose osteoporosis on the basis of the DXA scan alone. “We don't want to label children as having osteoporosis just on the basis of a bone density. That's really a clinical diagnosis based upon fractures,” Dr. Bachrach said.