What test confirms the diagnosis of a patient with chronic back pain and a history of consuming unpasteurized milk?

Brucella Titer Confirms the Diagnosis

In a patient with chronic back pain and a history of drinking unpasteurized milk, Brucella serology (Brucella titer) is the confirmatory test of choice. This clinical presentation is highly suggestive of brucellosis with spinal involvement (spondylitis or sacroiliitis), and serological testing is the primary diagnostic method for this condition 1, 2.

Why Brucella Titer is the Correct Answer

Clinical Context Points to Brucellosis

• Unpasteurized milk consumption is the classic risk factor for brucellosis transmission in the United States, making this the key epidemiological clue 1.
• Chronic back pain is a hallmark manifestation of brucellosis, which commonly causes spinal osteomyelitis, sacroiliitis, and spondylitis 1.
• The combination of these two features creates a high pre-test probability for brucellosis that warrants specific serological testing 1.

Serological Testing is the Primary Diagnostic Method

• Most diagnoses of acute brucellosis are made on the basis of positive serology, as blood culture sensitivity varies widely (15-70%) and may be negative in chronic cases 2, 3.
• Standard tube agglutination test (SAT) with titers ≥1/160 is diagnostic when combined with compatible clinical presentation 3.
• Brucella IgM antibodies appear early and are typically detectable at clinical presentation, with most patients showing increased titers by the second week of illness 2.
• Testing paired serum specimens 2-3 weeks apart to demonstrate four-fold or greater increase in antibody titer confirms acute infection 2.

Why Other Options Are Less Appropriate

ESR (Option B):

• While ESR is elevated in spine infections including brucellosis, it is non-specific and does not confirm the diagnosis 4.
• ESR is useful as a screening marker but requires specific testing to identify the causative organism 4.

Blood Culture (Option C):

• Blood culture is the gold standard when positive, but sensitivity is only 15-70% and may be negative in chronic cases 3.
• Bone marrow culture has higher sensitivity than blood culture but is more invasive 3.
• In the clinical setting of chronic back pain (suggesting established infection), blood cultures may already be negative while serology remains positive 2, 3.

MRI Spine (Option D):

• MRI would show structural changes (spondylitis, sacroiliitis, paraspinal abscess) but does not confirm the specific etiology 1.
• MRI is essential for evaluating extent of disease but requires microbiological confirmation of brucellosis 1.

Diagnostic Algorithm for This Patient

Initial Approach

1. Order Brucella serology immediately (SAT, Coombs test, or immunocapture agglutination test) 1, 3.
2. Obtain blood cultures before starting antibiotics to maximize yield, though they may be negative in chronic disease 4, 2.
3. Check inflammatory markers (ESR, CRP) as supportive evidence, expecting elevation in active infection 4.

Interpretation of Brucella Serology

• Titers ≥1/160 are diagnostic when combined with compatible clinical presentation 3, 5.
• If initial titer is lower, repeat testing in 2-3 weeks to demonstrate rising titers (four-fold increase confirms acute infection) 2.
• Coombs anti-Brucella test or immunocapture agglutination may be needed if SAT is negative despite high clinical suspicion, as blocking antibodies can cause false-negative SAT results 3, 5.

Imaging Considerations

• MRI of the spine should be performed to evaluate extent of spinal involvement (spondylitis, sacroiliitis, epidural abscess) once diagnosis is suspected 1, 4.
• Imaging guides treatment decisions but does not replace serological confirmation 1.

Common Pitfalls to Avoid

• Do not rely solely on blood cultures, as they have limited sensitivity especially in chronic brucellosis 3.
• Do not dismiss the diagnosis if initial SAT is negative—blocking antibodies can cause false negatives, requiring Coombs test or immunocapture agglutination for confirmation 3, 5.
• Do not delay serological testing while waiting for culture results, as serology is more sensitive and provides faster results 2, 3.
• Do not start antibiotics before obtaining blood cultures, as this further reduces culture sensitivity 4, 2.

Treatment Implications

Once brucellosis is confirmed serologically:

• Discuss treatment with an infectious disease specialist, as brucellosis requires prolonged combination antibiotic therapy 1.
• Standard treatment is doxycycline plus rifampin for 6 weeks for uncomplicated cases 1.
• Spinal involvement may require longer treatment duration and close monitoring with repeat inflammatory markers 4.