Osteomyelitis in Diabetic Patients

Diagnostics and Likelihood Ratios, Explained

Positive Findings (Patient Has This)

Signs/FindingsIncreased Disease Probability (Positive Likelihood Ratio)
Bone Exposure9.2× (0.57-146)
Ulcer Area > 2cm²7.2× (1.1-4.9)
Positive probe-to-bone test6.4× (3.6-11)
Clinical Gestalt5.5× (1.8-17)
Ulcer inflammation (erythema, swelling, purulence)1.5× (0.51-4.7)
Lab and Study FindingsIncreased Disease Probability (Positive Likelihood Ratio)
ESR > 70 mm/h11× (1.6-79.0)
Abnormal findings [indicating osteomyelitis] on plain Radiograph2.3× (1.56-3.3)
Swab Culture1× (0.65-1.5)

Negative Findings (Patient Doesn't Have This)

Signs/FindingsIncreased Disease Probability (Negative Likelihood Ratio)
Positive probe-to-bone test0.39× (0.20-0.76)
Ulcer Area > 2cm²0.48× (0.31-0.76)
Clinical Gestalt0.54× (0.30-0.97)
Bone Exposure0.70× (0.53-0.92)
Ulcer inflammation (erythema, swelling, purulence)0.84× (0.56-1.3)
Lab and Study FindingsDecreased Disease Probability (Negative Likelihood Ratio)
ESR > 70 mm/h0.34× (0.06-1.9)
Abnormal findings [indicating osteomyelitis] on plain Radiograph0.63× (0.51-0.78)
Swab Culture1× (0.08-13)


  • Outcome: osteomyelitis

  • Conclusion: "No single historical feature or physical examination reliably
    excludes osteomyelitis."


Studies were retro- or prospective; mostly level III or IV evidence; most patients had foot ulcers, or infection, or suspected osteomyelits; all tests used gold standard (bone biopsy w histopathology diagnosis, bone culture or both) for comparison; In- or out-patient location was not clear in most cases; likely strong selection bias.

Not all criteria for evaluation of osteomyelitis on radiographic examination were reported in each study.


Rodrigo Kong, MD and Shahriar Zehtabchi, MD


February 15, 2013

What are Likelihood Ratios?

LR, pretest probability and posttest (or posterior) probability are daunting terms that describe simple concepts that we all intuitively understand.

Let's start with pretest probability: that's just a fancy term for my initial impression, before we perform whatever test it is that we're going to use.

For example, a patient with prior stents comes in sweating and clutching his chest in agony, I have a pretty high suspicion that he's having an MI – let's say, 60%. That is my pretest probability.

He immediately gets an ECG (known here as the "test") showing an obvious STEMI.

Now, I know there are some STEMI mimics, so I'm not quite 100%, but based on my experience I'm 99.5% sure that he's having an MI right now. This is my posttest probability - the new impression I have that the patient has the disease after we did our test.

And likelihood ration? That's just the name for the statistical tool that converted the pretest probability to the posttest probability - it's just a mathematical description of the strength of that test.

Using an online calculator, that means the LR+ that got me from 60% to 99.5% is 145, which is about as high an LR you can get (and the actual LR for an emergency physician who thinks an ECG shows an obvious STEMI).

(Thank you to Seth Trueger, MD for this explanation!)