Hairy Cell Leukemia: Diagnosis and Tests

Hairy cell leukemia often causes gradual symptoms that build slowly. These include fatigue, weakness, frequent infections, and easy bruising or bleeding. Some people notice shortness of breath, or a feeling of fullness below the ribs, from an enlarged spleen. Because it progresses slowly, this leukemia is sometimes found on a routine blood test before symptoms appear.

A complete blood count checks your red cells, white cells, and platelets, and often shows low counts. A peripheral blood smear lets a pathologist look directly at your blood cells under a microscope. They search for cells with the fine, hair-like projections that give this leukemia its name. A bone marrow biopsy confirms the diagnosis and shows how much marrow is involved. Immunophenotyping, which reads specific markers on the cell surface, distinguishes hairy cell leukemia from other, similar-looking blood cancers.

Most people with hairy cell leukemia have a change in a gene called BRAF. The specific change is called BRAF V600E. This is not something you inherited; it developed only in the leukemia cells. Finding it helps confirm the diagnosis. It also opens the door to targeted pills, called BRAF inhibitors, if treatment becomes necessary later.

Hairy cell leukemia often causes low blood counts. That does not automatically mean treatment must start immediately. Many people are simply watched for a period, since this leukemia tends to move slowly. Ask your team what specific counts or symptoms would trigger starting treatment. Do not assume any abnormal number means urgent action.

Turnaround depends on the test and on the lab. For the bone marrow sample, the National Cancer Institute says the pathologist typically sends a report to your doctor within 10 days of the biopsy, since preparing the slides takes several days. Timing for blood counts and for BRAF gene testing varies, so ask your team when each result is due and who will contact you.

• What do my blood counts show, and how do they compare to prior results?
• Was BRAF V600E testing done, and what was the result?
• Am I a candidate for watchful waiting, or does treatment need to start soon?
• Is my spleen enlarged, and does that change anything?
• What symptoms should prompt me to call between visits?

• Call 911 or go to an emergency department if bleeding will not stop despite firm, steady pressure, you bring up blood or pass black stools, or you become suddenly and severely breathless. Low platelet and red cell counts are common in hairy cell leukemia, so bleeding and breathlessness are taken seriously.
• Call 911 or go to an emergency department if your temperature reaches 100.4°F (38°C) or higher, or you get shaking chills, while your white count is low or you are having treatment. CDC treats a fever in this situation as a medical emergency: it may be the only sign of an infection, and that infection can become life-threatening fast. Say at the door that you have leukaemia and may be neutropenic.
• Call your care team the same day if you get a sore throat, a cough, or burning when you pass urine without a fever, or redness or oozing around any line site.
• Call your care team within a day or two if bruises appear without a knock, tiredness deepens noticeably, or the fullness or ache below your ribs is growing.

• National Cancer Institute — Hairy Cell Leukemia Treatment (PDQ).
• National Cancer Institute — Infection and Neutropenia during Cancer Treatment
• CDC — Fever and Cancer Treatment, accessed August 13, 2026.
• National Cancer Institute — Pathology Reports