A brief trial of corticosteroids can be diagnostic for allergic causes, as the eosinophilia should resolve with suppression of the immune over-response. Neoplastic disorders are diagnosed through the usual methods, such as bone marrow aspiration and biopsy for the leukemias, MRI/CT to look for solid tumors, and tests for serum LDH and other tumor markers. Treatment
Treatment is directed toward the underlying cause. However, in primary eosinophilia, or if the eosinophil count must be lowered, corticosteroids such as prednisone may be used. However, immune suppression, the mechanism of action of corticosteroids, can be fatal in patients with parasitosis. List of causes
Eosinophilia can be idiopathic (primary) or, more commonly, secondary to another disease. In the Western World, allergic or atopic diseases are the most common causes, especially those of the respiratory or integumentary systems. In the developing world, parasites are the most common cause. A parasitic infection of nearly any bodily tissue can cause eosinophilia. Diseases that feature eosinophilia as a sign include:

Allergic disorders
Asthma
Hay fever
Drug allergies
Allergic skin diseasesPemphigus
Dermatitis herpetiformis
IgG4-related disease
Parasitic infections
Addisons disease and stress-induced suppression of adrenal gland function
Some forms of malignancy
Acute lymphoblastic leukemia
Chronic myelogenous leukemia
Eosinophilic leukemia
Clonal eosinophilia
Hodgkin lymphoma
Some forms of non-Hodgkin lymphoma
Lymphocyte-variant hypereosinophilia
Systemic mastocytosis
Systemic autoimmune diseasesSystemic lupus erythematosus
Kimura disease
Eosinophilic granulomatosis with polyangiitis
Eosinophilic fasciitis
Eosinophilic myositis
Eosinophilic myocarditis
Eosinophilic esophagitis
Eosinophilic gastroenteritis
Cholesterol embolism (transiently)
Coccidioidomycosis (Valley fever), a fungal disease prominent in the US Southwest. Human immunodeficiency virus infection
Interstitial nephropathy
Hyperimmunoglobulin E syndrome, an immune disorder characterized by high levels of serum IgE
Idiopathic hypereosinophilic syndrome.
While virtually any drug should be considered as a possible cause of these signs and symptoms, the following drugs and drug classes are some of the most frequently reported causes: penicillins, cephalosporins, dapsone, sulfonamides, carbamazepine, phenytoin, lamotrigine, valproic acid, nevirapine, efavirenz, and ibuprofen. These drugs may cause severely toxic reactions such as the DRESS syndrome. Other drugs and drug classes often reported to cause increased blood eosinophil levels accompanied by less severe (e.g. non-DRESS syndrome) symptoms include tetracyclins, doxycycline, linezolid, nitrofurantoin, metronidazole, carbamazepine, phenobarbital, lamotrigine, valproate, desipramine, amitriptyline, fluoxetine, piroxicam, diclofenac, ACE inhibitors, abacavir, nevirapine, ranitidine, cyclosporin, and hydrochlorothiazide.The toxic oil syndrome is associated with hypereosinophilia/eosinophilia and systemic symptoms due to one or more contaminants in rapeseed oil and the Eosinophilia–myalgia syndrome, also associated with hypereosinophilia, appears due to trace contaminants in certain commercial batches of the amino acid, L-tryptophan.Allergic reactions to drugs are a common cause of eosinophilia, with manifestations ranging from diffuse maculopapular rash, to severe life-threatening drug reactions with eosinophilia and systemic symptoms (DRESS). Drugs that has, allopurinol, nonsteroidal anti-inflammatory drugs (NSAIDs), some antipsychotics such as risperidone, and certain antibiotics. Phenibut, an analogue of the neurotransmitter GABA, has also been implicated in high doses. The reaction which has been shown to be T-cell mediated may also cause eosinophilia-myalgia syndrome. Malignancies
Certain malignancies cause a secondary eosinophilia or, less commonly, hypereosinophilia. These increases in blood eosinophils appear due to the release of stimulatory cytokines or invasion of the bone marrow and thereby irritation of resident eosinophils or their precursors.