Cath Lab Digest

Dr. Armstrong: Let’s talk about more of our diagnostic modalities. What about some of the things that have been talked about as gold standards in diagnosing deep infections such as osteomyelitis? What about probing to bone for instance? Is it an accurate means of assessing osteomyelitis? Is it worthwhile? Should we be doing it in all of our patients?
Dr. Lipsky: One study described using this technique for diagnosing osteomyelitis. Because the positive predictive value of the test was 89 percent and it’s cheap and easy to do, people have widely adopted it as a means of diagnosing osteomyelitis, without going any further.²
Clinicians need to understand that there are some caveats to interpreting the results of that study. It took place at one center and the probing was done by only one or two people. Secondly, all of those patients were enrolled in a study of antimicrobial therapy that was looking at people with limb-threatening infection. Because of that, the pre-test probability of osteomyelitis in that group of patients was 66 percent. Therefore, the positive predictive value of any test would be pretty good and probably overstated in this population.

The next problem is that the test is often not performed as described by practitioners. I often see people say they’re probing to bone using the back of a wooden swab. The feel of bone under a wooden swab is not the same as it is under a metal probe.
The final issue with this diagnostic modality is to ask about the inter- and intra-observer variability of the test. If the four of us were to do this test blinded on a patient, would we all agree that it was positive or negative? Even if I was to do the test on the same patient more than once, would I agree with myself?

What Studies Reveal About The Probe To Bone Test
With these questions in mind, we put the probe to bone method to test in a study that we recently completed, comparing linezolid against an aminopenicillin beta-lactamase inhibitor combination.³
That study was conducted at multiple centers on 370 patients using the technique, as defined in the original paper, with a metal 14-gauge eye probe. The prevalence of osteomyelitis in our patients was only a third of that in the original paper. We hope to report our results soon. In the meantime, I view the probe to bone as a useful test, but not predictive enough to stop doing other diagnostic tests if the rest of the clinical picture is not highly suggestive of osteomyelitis.

Addressing Other Misconceptions About The Test
Dr. Armstrong: So it’s possible that the more high-risk patients you are seeing, the greater the likelihood that this probe to bone test will in fact be positive and potentially useful. However, perhaps for many clinicians who are in private practice and who are not seeing these osteomyelitic patients on a daily basis being referred to them, the pre-test probability is lower. Therefore, there’s a lower likelihood that this probe to bone test will be a definitive diagnostic test for osteomyelitis.
We sometimes bandy about the notion that these probes are a “poor man’s bone scan.” I think that’s probably a bit injudicious. How then should we go forward in diagnosing bone infections complicating soft tissue infections? If we can’t probe to bone, are we going to get a bone scan or a Ceretec scan or an Indium 111 or an MRI?
Dr. Joseph: People always come up to me and say they had this case in which they could touch the bone or see the bone. Therefore, they said they had to treat it for osteomyelitis. I ask them exactly what they were using to probe. But I always liked the old podiatry packer-spatula. It works wonderfully. It works as well as the eye probe.
Dr. Lipsky: But to the extent that the test was defined with a certain technique, you can’t really say that you can use a non-validated technique and had a positive probe to bone. It may well be that there are other devices that are more effective, but until we’ve tested them, it’s difficult to say …
Dr. Joseph: Right. It just goes to exactly what you were talking about earlier. We don’t have any of these questions answered. Does it make a difference if you use a 14-gauge eye probe vs. a packer-spatula? I agree with you that the wood feel is going to blunt it.