Eosinophil Count of 1.3 × 10⁹/L in an Asymptomatic Patient
An eosinophil count of 1.3 × 10⁹/L represents moderate eosinophilia that requires systematic evaluation to exclude helminth infections (particularly in travelers), medication causes, and atopic conditions, even in the absence of symptoms, because this level can be associated with subclinical end-organ damage and may progress to serious complications. 1
Understanding the Significance of This Count
Your count of 1.3 × 10⁹/L falls into the "moderate eosinophilia" category (defined as 0.5-1.5 × 10⁹/L), which is never explained by allergy alone and always requires comprehensive workup to exclude secondary causes. 1, 2
This level carries potential risk because hypereosinophilia ≥1.5 × 10⁹/L persisting for more than 3 months, or counts exceeding 5.0 × 10⁹/L at any time, carries significant risk of morbidity and mortality from end-organ damage. 1 Your count is approaching this threshold.
The absence of symptoms does not exclude the possibility of subclinical organ involvement, as end-organ damage can be initially asymptomatic. 1
Immediate Evaluation Required
Critical History Elements to Assess
Document any travel history to tropical or helminth-endemic regions, as helminth infections account for 19-80% of eosinophilia cases in returning travelers or migrants. 3, 1, 2 Geographic exposure details should include timing relative to eosinophilia onset, as eosinophilia typically appears 4-12 weeks post-exposure during tissue migration phases. 2
Review all medications started within the past 3 months, as drug-induced eosinophilia is a common non-infectious cause. 3, 2 Common culprits include NSAIDs, beta-lactam antibiotics, and nitrofurantoin. 3
Assess for atopic history including asthma, eczema, and allergic rhinitis, which account for approximately 80% of eosinophilia cases in non-tropical populations. 2 However, mild eosinophilia (0.5-1.5 × 10⁹/L) in non-endemic areas is most commonly caused by allergic disorders or medications, but in returning travelers or migrants, helminth infections remain the leading identifiable cause. 1
Inquire about freshwater exposure in Africa or tropical regions, raw or undercooked meat consumption, and any gastrointestinal symptoms (even mild). 1, 2
Essential Laboratory Workup
The following tests should be performed immediately:
Three separate concentrated stool specimens for ova and parasites on different days, regardless of symptoms, as helminth infection causes 14-64% of eosinophilia in returning travelers. 3, 2
Strongyloides serology is essential for all patients given the risk of fatal hyperinfection syndrome in immunocompromised hosts, even >50 years after exposure. 1, 2 This is critical because Strongyloides stercoralis can persist lifelong and cause fatal hyperinfection syndrome in immunocompromised patients with mortality rates approaching 70%. 1, 2
Schistosomiasis serology should be performed if freshwater exposure in endemic areas (particularly Africa) within the past 4-8 weeks. 1, 2
HIV testing should be considered, as HIV infection may present with eosinophilia, although helminth co-infection is still more likely. 3
Important Clinical Context
Why This Matters Even Without Symptoms
Normal eosinophil counts do not exclude parasitic infection, as many helminth-infected patients do not have eosinophilia. 1 Conversely, only tissue-invasive helminthic parasites cause eosinophilia, limiting its application as a general screening tool. 1
Long-standing hypereosinophilia can cause significant end-organ damage affecting heart, lungs, CNS, and skin. 2 Specifically, eosinophilic myocarditis is the number one cause of morbidity and mortality in hypereosinophilic syndrome, presenting in 20% of cases. 2
Schistosomiasis can cause spinal cord compression or portal hypertension in chronic cases, and Schistosoma haematobium is associated with squamous cell bladder carcinoma. 1
Seasonal and Demographic Factors
Substantial seasonal differences in eosinophil counts occur, with differences of approximately 20% between July and January. 4 This means your count may vary naturally throughout the year.
Male sex is associated with 14% higher eosinophil counts in adults, and obesity is associated with 19% higher counts. 5 Current smoking increases counts by 17%. 5
Nasal polyps increase eosinophil levels by 38%, whereas current smoking decreases levels by 23%. 4
Follow-Up and Monitoring Strategy
If Initial Workup is Negative
If eosinophilia persists ≥1.5 × 10⁹/L for more than 3 months without identified cause after treatment or exclusion of infectious causes, refer to hematology. 1, 2
A single measurement may not be sufficient when using eosinophil counts for diagnosis/management, as eosinophil levels between 150-299 cells/μL (0.15-0.3 × 10⁹/L) are least stable. 4 Your count of 1.3 × 10⁹/L is in a more stable range, but repeat measurement in 4-6 weeks is advisable to assess persistence.
Screening for Subclinical Organ Damage
If your count remains elevated or you develop any symptoms, the following assessments may be warranted:
Electrocardiogram, cardiac troponin, and NT-proBNP measurement to screen for myocardial injury. 1
Chest X-ray to identify pulmonary infiltrates if any respiratory symptoms develop. 1
Pulmonary function tests if respiratory symptoms such as persistent cough, wheezing, or dyspnea occur. 1
Critical Pitfalls to Avoid
Do not assume eosinophilia alone is adequate screening for helminth infection, as many infected patients have normal eosinophil counts. 1
Do not wait for symptoms to develop before investigating persistent moderate-to-severe eosinophilia, as end-organ damage can be subclinical initially. 1
Do not use diethylcarbamazine (DEC) if Loa loa microfilariae are seen on blood film, as it may cause fatal encephalopathy; use corticosteroids with albendazole first to reduce microfilarial load before definitive treatment. 1
Do not ignore the possibility of eosinophilic esophagitis if any gastrointestinal symptoms develop, as peripheral eosinophilia occurs in only 10-50% of adults with this condition, and tissue biopsy remains the gold standard. 3, 1