If you’ve been following along with parts 1 and 2, then part 3 is the next logical step- because, um, counting?
Let’s look at laboratory studies in osteo. It may come as no surprise that the initial labs that end up being most helpful are CRP, ESR and blood culture. Pääkkönen et al noted that both ESR and CRP were highly sensitive at levels above 20 mm/hr for ESR and 2 mg/dL for CRP were 94% and 05% senistive respectively. If both were elevated, which almost always happens within the first 3 days of what will eventually become culture positive osteo, the sensitivity is 98%. In their study, CBC was less sensitive though we still often seem to get it perhaps reflexively. It now seems that CBCs don’t contribute to the diagnosis of osteo like the CRP and ESR. However, if you are trying to rule out leukemia because of bone pain by all means get the CBC. The choice, as always depends on your index of suspicion and level of comfort. Although the WBC count may reach a level of 20,000 per mm3, it falls within the normal range in two-thirds of cases. Procalcitonin was highly unlikely to be elevated in conditions other than osteo in a study by Butbol-Aviel et al. It doesn’t offer distinct advantages to CRP, and thus I would still only consider it as adjunctive at this time, especially since trending CRP as it declines suggests favorable response. Procalcitonin it seems is still waiting for its seat at the table. In cases of MRSA osteo Ju et al. found that, in contrast to MSSA, the resistant had four significant independent multivariate predictors:
- Temperature of >38°C
- Hematocrit value of <34%
- WBC >12,000 cells/μL
- CRP >1.3 mg/dL
If the child had 4/4 they had a 92% probability of MRSA. With 3 it was 45%, 10% for two, 1% for one, and 0% for none. This study seems to support getting a CBC, especially if you work where MRSA does.
When we return to part 4 we’ll look at imaging
