Viral infection: A low neutrophil count (neutropenia) can be seen in various viral infections, which might not necessarily cause an elevation in C-reactive protein (CRP) levels. The body's response to viral infections can vary, and a mild decrease in white blood cell count (leukopenia) with neutropenia is plausible.

Bone marrow suppression: Certain medications, chemicals, or medical conditions can suppress the bone marrow, leading to decreased production of blood cells, including neutrophils. A negative CRP suggests that there might not be an acute bacterial infection, but bone marrow suppression could still be a cause of the low neutrophil count.
Chronic idiopathic neutropenia: This condition is characterized by an unexplained decrease in neutrophil count that persists over time. It might not be associated with an elevated CRP, especially if there's no concurrent infection.
Hypersplenism: An overactive spleen can sequester neutrophils, leading to a low count in the peripheral blood. This condition might not directly affect CRP levels unless there's an underlying infection or inflammation.

Severe combined immunodeficiency (SCID): Although rare, SCID is a condition where the immune system is severely impaired, leading to low counts of various immune cells, including neutrophils. It's crucial to consider this diagnosis, especially in infants or young children presenting with recurrent severe infections.
Aplastic anemia: A condition where the bone marrow fails to produce blood cells, including neutrophils. It's essential to diagnose and treat aplastic anemia promptly to prevent severe complications.
Felty's syndrome: A rare condition associated with rheumatoid arthritis, characterized by neutropenia, splenomegaly, and sometimes rheumatoid nodules. Although CRP might be negative, the clinical context and other inflammatory markers could suggest this diagnosis.

Cyclic neutropenia: A rare condition where neutrophil counts fluctuate cyclically, leading to periods of severe neutropenia. This diagnosis should be considered if there's a pattern of recurring neutropenia.
Kostmann syndrome: A rare congenital disorder characterized by severe neutropenia. It's essential to diagnose this condition early to manage the risk of severe infections.
Autoimmune neutropenia: An immune system disorder where the body produces antibodies against its own neutrophils, leading to their destruction. This condition might present with a low neutrophil count and could be considered if other causes are ruled out.