What is the most likely diagnosis in a patient with mildly elevated C-reactive protein (13.8 mg/L), erythrocyte sedimentation rate (39 mm/hr), negative antinuclear antibody (ANA) test, and recurrent neck swelling?

Most Likely Diagnosis: Infectious Lymphadenitis or Polymyalgia Rheumatica

The most likely diagnosis in this patient is infectious lymphadenitis, particularly if the patient is younger, or polymyalgia rheumatica (PMR) if the patient is over 50 years old. The mildly elevated inflammatory markers (CRP 13.8 mg/L, ESR 39 mm/hr) with negative ANA and recurrent neck swelling point toward either an infectious process or a seronegative inflammatory condition rather than autoimmune connective tissue disease.

Diagnostic Reasoning

Why Infection is High on the Differential

The pattern of mildly elevated CRP with recurrent neck swelling strongly suggests infectious lymphadenitis 1. Research shows that infections, particularly those involving lymph nodes and bone/joint structures, commonly present with elevated inflammatory markers and localized swelling 1. The negative ANA effectively rules out most systemic autoimmune conditions like systemic lupus erythematosus, which would be less likely to present with isolated cervical lymphadenopathy anyway 2.

Age-Dependent Considerations

If your patient is over 50 years old, polymyalgia rheumatica becomes the leading diagnosis. In patients presenting with nonspecific elevations in ESR/CRP and no prior rheumatic disease, PMR accounts for 38% of new-onset rheumatic diseases, making it the single most common etiology in this demographic 3. PMR can present with neck pain and stiffness that might be interpreted as "swelling."

If your patient is younger (under 50), consider:

• Bacterial lymphadenitis (most common)
• Kawasaki disease (if pediatric with fever ≥5 days)
• Seronegative rheumatoid arthritis (second most common new-onset rheumatic disease) 3

The Inflammatory Marker Pattern

Your patient's CRP of 13.8 mg/L is only mildly elevated. CRP levels are significantly higher in infections compared to new-onset rheumatic diseases or malignancies 3. However, this relatively modest elevation doesn't exclude infection—it simply suggests a less severe or more chronic/recurrent process rather than acute bacterial infection.

The ESR of 39 mm/hr is also mildly elevated. These concordant but modest elevations are typical of:

• Subacute or chronic infections
• Inflammatory rheumatic conditions
• Resolving acute inflammation 4

Critical Next Steps

Imaging is Essential

Obtain neck ultrasound or CT imaging immediately to differentiate between infectious lymphadenitis and other causes 5. Key imaging findings:

• Bacterial lymphadenitis: Single enlarged node with hypoechoic core
• Kawasaki disease (if pediatric): Multiple enlarged lymph nodes with retropharyngeal edema or non-suppurative phlegmon 5
• Malignancy: Concerning features include fixed masses, matted nodes, or abnormal architecture

Clinical Features to Assess

Look specifically for:

Infectious signs:

• Fever pattern (persistent vs. intermittent)
• Unilateral vs. bilateral neck swelling
• Tenderness, warmth, erythema over swelling
• Recent dental work or pharyngitis
• Response to prior antibiotics (if any were given)

Kawasaki disease criteria (if pediatric with fever ≥5 days) 5:

• Bilateral conjunctival injection without exudate
• Oral changes (cracked lips, strawberry tongue)
• Polymorphous rash
• Hand/foot erythema or edema
• Cervical lymphadenopathy ≥1.5 cm

PMR features (if age >50):

• Bilateral shoulder and hip girdle pain/stiffness
• Morning stiffness >45 minutes
• Difficulty with overhead activities
• Constitutional symptoms

Common Pitfalls to Avoid

1. Don't dismiss infection because CRP is "only" mildly elevated—chronic or localized infections can present with modest inflammatory marker elevations 3

2. Don't assume negative ANA rules out all rheumatic disease—seronegative rheumatoid arthritis and PMR are both ANA-negative conditions that commonly present with elevated inflammatory markers 3

3. Don't delay imaging in recurrent neck swelling—persistent or recurrent lymphadenopathy warrants imaging to rule out malignancy, which accounts for 9.2% of cases with nonspecific ESR/CRP elevations 3

4. In pediatric patients with persistent fever and neck swelling, maintain high suspicion for Kawasaki disease—cervical lymphadenopathy may be the most prominent initial finding, and delayed diagnosis increases risk of coronary artery complications 5

Malignancy Consideration

While less likely given the modest inflammatory marker elevations, malignancy accounts for approximately 9% of cases presenting with elevated ESR/CRP and nonspecific findings 3. The recurrent nature of symptoms and negative ANA make this worth excluding with imaging, particularly if the patient has constitutional symptoms (weight loss, night sweats, fatigue).