To diagnose Neutropenia, the doctor may order a complete blood count to determine the number of neutrophils in the blood. Sometimes, to make a precise diagnosis especially if the doctor suspects other serious illnesses, a bone marrow aspiration or biopsy may be necessary.

In a bone marrow biopsy, the doctor will use a needle to take a sample of the patient's bone marrow and examine it under a microscope. The doctor will be looking for impaired neutrophil production, or if there has been excessive use or destruction of these cells. This test will show if there are any signs of an intrinsic defect in the marrow, Congenital Neutropenia, maturation arrest, a fungal infection, a lack of Vitamin B12 or a folate deficiency in the body. There may be other diseases present, as well, like leukemia, tuberculosis, and other cancers.

When the patient exhibits a form of Neutropenic fever, laboratory tests like blood cultures, urinalysis, Sputum Gram stain and culture, and measuring the serum Vitamin B12 and RBC folate levels may be recommended.

For patients already afflicted with an infection, taking radiograph images of the posterio-anterior and lateral chests may be required to check for any signs of pneumonia.