Diagnosis of vertebral osteomyelitis (VO)

Reference: JBJI review published on 27/1/2022. Read the full paper here – Maamari, J., Tande, A. J., Diehn, F., Tai, D. B. G., and Berbari, E. F.: Diagnosis of vertebral osteomyelitis, J. Bone Joint Infect., 7, 23–32, https://doi.org/10.5194/jbji-7-23-2022, 2022.

Biomarkers

• CRP and ESR have the highest sensitivity (94 %–100 %), especially when used together.
• White cell count has low sensitivity and specificity.
• >90% of patients with VO have an ESR of >50. ESR is usually more elevated in bacterial VO compared to TB VO.
• If fever and MRI findings, the predictive value may increase further.

Imaging

• MRI (preferably with intravenous gadolinium contrast) is the preferred mode of imaging with high sensitivity (97%), specificity (92%), and accuracy (94%). MRI can identify marrow oedema as early as 48 hrs. However, MRI can be repeated in 2-4 weeks if the early imaging is inconclusive. Routine repeat MRI to test response may not help as the radiological response lags behind the clinical response.
• X-ray – low sensitivity at the early stage of the disease (abnormalities appear 2 to 8 weeks after the onset of symptoms), but can be used as the initial test to rule out other pathologies and spinal enumeration. Comparison with old X-rays may help to identify subtle signs.
• CT scan – CT could be superior to MRI in some cases -Modic type endplate changes and if clinical findings do not strongly suggest infection.
• Nuclear imaging –
  – Scintigraphy with single-photon emission computed tomography (SPECT) using Technetium-99m (99mTc) and Gallium-67 (67Ga) tracers can be used when MRI is contraindicated.
  – Positron emission tomography–computed tomography (PET/CT) also can be used and may have similar efficacy to MRI.

Biopsy and Microbiology

• Blood culture: positive in 58% of cases (range 30-70%). Two sets of blood cultures are recommended. Previous antibiotic therapy may affect the blood culture positivity. If a typical organism is identified (St aureus, St lugdunensis, Brucella) – further investigation is not necessary.
• Biopsy – when blood or other cultures does not give a conclusive result, image-guided (typically CT or fluoroscopy) percutaneous biopsy or open biopsy is necessary. Open biopsy has higher yield (76%) compared to percutaneous (48-52%). Biopsy specimens should be sent for culture and histopathology.
• Factors which increase the yield are – an elevated CRP, a lower-gauge needle, an increased number of specimens and aspiration of a fluid collection.
• Repeat biopsy: if the initial needle biopsy is not diagnostic, a repeat needle biopsy can be undertaken after 3 days, or an open biopsy can be done.

Serology

• Fungal serology can be considered, if risk factor present, endemic situation or clinically suspected.
• Brucella serology: if Brucella is suspected – Blood culture and serology should be used for diagnosis. A cutoff of > 1 : 160 for Brucella antibodies or > 1 : 320 for the Coombs test is considered positive.
• TB: a purified protein derivative test or an interferon-γ release assay could be used for their high negative predictive value.

Novel modalities

• Low-tesla open-magnet MRI scanners – 86 % sensitivity with a 100 % specificity for MRI-guided biopsies.
• 16S ribosomal RNA (rRNA) PCR and TB PCR to support microbiological diagnosis.
• Metagenomic modalities – Metagenomic next-generation sequencing (mNGS) – is complementary to culture-based methods.

Flow chart as shown in the above article