Hairy cells are visible in this test in about 85% of cases.Most patients require a bone-marrow biopsy for final diagnosis. The bone marrow biopsy is used both to confirm the presence of HCL and also the absence of any additional diseases, such as Splenic marginal zone lymphoma or B-cell prolymphocytic leukemia. The diagnosis can be confirmed by viewing the cells with a special stain known as TRAP (tartrate resistant acid phosphatase). More recently, DB44 testing assures more accurate results. Definitively diagnosing HCL is also possible through flow cytometry on blood or bone marrow. The hairy cells are larger than normal and positive for CD19, CD20, CD22, CD11c, CD25, CD103, and FMC7 antigens. (CD103, CD22, and CD11c are strongly expressed. )Hairy cell leukemia-variant (HCL-V), which shares some characteristics with B cell prolymphocytic leukemia (B-PLL), does not show CD25 (also called the interleukin-2 receptor, alpha). The differential diagnoses include several kinds of anemia, including myelophthisis and aplastic anemia, and most kinds of blood neoplasms, including hypoplastic myelodysplastic syndrome, atypical chronic lymphocytic leukemia, B-cell prolymphocytic leukemia, or idiopathic myelofibrosis. Classification
When not further specified, the "classic" form is often implied, but two variants have been described:  Hairy cell leukemia-variant and a Japanese variant. The non-Japanese variant is more difficult to treat than either classic HCL or the Japanese HCL. HCL-V
Hairy cell leukemia-variant (HCL-V) is usually described as a prolymphocytic variant of HCL. It was first formally described in 1980 by a paper from the University of Cambridges Hayhoe laboratory.
Interferon-alpha is an immune system hormone that is very helpful to a relatively small number of patients, and somewhat helpful to most patients. In about 65% of patients, the drug helps stabilize the disease or produce a slow, minor improvement for a partial response.The typical dosing schedule injects at least 3 million units of interferon-alpha (not pegylated versions) three times a week, although the original protocol began with 6 months of daily injections. Some patients tolerate IFN-alpha very well after the first fw weeks, while others find that its characteristic flu-like symptoms persist. About 10% of patients develop a level of depression. By maintaining a steadier level of the hormone in the body, daily injections might cause fewer side effects in selected patients. Drinking at least 2 liters of water each day, while avoiding caffeine and alcohol, can reduce many of the side effects. A drop in blood counts is usually seen during the first 1–2 months of treatment. Most patients find that their blood counts get worse for a few weeks immediately after starting treatment, although some patients find their blood counts begin to improve within just 2 weeks.Typically, 6 months are neededto figure out whether this therapy is useful. Common criteria for treatment success include:

Normalization of hemoglobin levels (above 12.0 g/dL),
A normal or somewhat low platelet count (above 100 K/µL), and
A normal or somewhat low absolute neutrophil count (above 1.5 K/µL).If it is well tolerated, patients usually take the hormone for 12 to 18 months.