The ICD 10 code for chronic lymphocytic leukemia is C91.1, “chronic lymphocytic leukemia of B-cell type.” In the US version (ICD-10-CM) it is extended to a billable fifth character that records disease status: C91.10 (not having achieved remission), C91.11 (in remission), and C91.12 (in relapse). Choosing the right one depends on where the patient is in their illness, so it helps to understand both the coding rules and the disease behind them.
Below I explain the codes, the common coding pitfalls, and the clinical picture of chronic lymphocytic leukemia (CLL) that clinicians document to support them.
The ICD-10 Codes for CLL at a Glance
ICD-10 is the international classification used to record diagnoses on medical records, insurance claims, and cancer registries. CLL sits in the C91 category, “lymphoid leukemia,” alongside related but distinct conditions.
| Code | Description | When it is used |
|---|---|---|
| C91.1 | Chronic lymphocytic leukemia of B-cell type | Category heading (WHO ICD-10); not billable alone in ICD-10-CM |
| C91.10 | CLL not having achieved remission | New diagnosis, watch and wait, or active disease on treatment |
| C91.11 | CLL in remission | Clinician documents remission after treatment |
| C91.12 | CLL in relapse | Disease has returned after a documented remission |
| Z85.6 | Personal history of leukemia | Only when the leukemia is no longer present or treated |
Other C91 codes cover different diseases, including acute lymphoblastic leukemia (C91.0), prolymphocytic leukemia of B-cell type (C91.3), and hairy cell leukemia (C91.4). They should not be used for CLL.
Coding Tips and Common Pitfalls
CLL versus small lymphocytic lymphoma
Small lymphocytic lymphoma (SLL) is the same disease as CLL, but located mainly in lymph nodes rather than blood. In ICD-10-CM, SLL is coded under the lymphoma category C83.0, with a final character for the lymph node site involved. Coders should follow the diagnosis the clinician documents: “CLL” goes to C91.1x, “SLL” to C83.0x.
Remission must be documented
Remission status is a clinical judgment. A coder should not assign C91.11 simply because blood counts look normal; the treating physician must state that the disease is in remission. If the record is silent, the default is C91.10.
Active disease versus history
Because CLL is usually a chronic, long-term condition, patients who are being monitored without treatment still have active disease and are coded C91.10, not Z85.6. The history code applies only when the leukemia has been eradicated and no further treatment is directed at it.
Transformation
If CLL transforms into an aggressive lymphoma, known as Richter transformation, the new lymphoma is coded separately according to its type, usually diffuse large B-cell lymphoma.
What Is Chronic Lymphocytic Leukemia?
CLL is a cancer of B lymphocytes, white blood cells that normally produce antibodies. It starts in the bone marrow, where abnormal but mature-looking B cells accumulate slowly. Over time they build up in the blood, marrow, lymph nodes, and spleen.
It is the most common leukemia in adults in Western countries and mainly affects older people. Unlike acute leukemias, CLL often progresses over years, and many people live with it for a long time without needing treatment. For a wider view of related conditions, see our overview of hematologic disorders.
Causes and risk factors
The exact cause is unknown. The strongest known risk factors are older age and a family history of CLL or related lymphoid cancers in first-degree relatives. Recurrent genetic changes in CLL cells include deletion 13q14, trisomy 12, deletion 11q, and deletion 17p, as well as TP53 mutations and the mutation status of the IGHV gene. These features help predict how the disease will behave.
Symptoms, Diagnosis, and Staging
Many people are diagnosed by chance when a routine blood test shows lymphocytosis, a raised lymphocyte count. When symptoms occur, they can include:
- Painless swollen lymph nodes in the neck, armpits, or groin
- Fatigue
- Night sweats, fevers, or unintentional weight loss (“B symptoms”)
- Fullness under the left ribs from an enlarged spleen
- Frequent infections due to weakened immunity
Diagnosis requires at least 5 × 10^9/L clonal B lymphocytes in the blood, confirmed by flow cytometry. The typical CLL pattern is B-cell markers (CD19, CD20 at low intensity) together with CD5 and CD23, and a single light-chain type. A bone marrow biopsy is not always needed for diagnosis. FISH testing and TP53/IGHV analysis are usually done before treatment starts, because they guide drug choice.
Two staging systems are used worldwide:
| System | Early | Intermediate | Advanced |
|---|---|---|---|
| Rai | Stage 0: lymphocytosis only | Stages I–II: enlarged nodes, spleen, or liver | Stages III–IV: anemia (Hb below 11 g/dL) or platelets below 100 × 10^9/L |
| Binet | A: fewer than 3 enlarged areas | B: 3 or more enlarged areas | C: anemia (Hb below 10 g/dL) or platelets below 100 × 10^9/L |
Treatment and Ongoing Management
Early-stage CLL without symptoms is usually managed with active monitoring, often called “watch and wait.” Starting treatment early in this group has not been shown to help, so regular check-ups and blood counts are the standard approach. Treatment begins when there are symptoms, bulky or growing nodes, or falling blood counts.
Current options include:
- BTK inhibitors such as ibrutinib, acalabrutinib, and zanubrutinib, taken by mouth, which block B-cell receptor signaling
- BCL-2 inhibitors such as venetoclax, usually combined with an anti-CD20 antibody for a fixed duration
- Chemoimmunotherapy such as fludarabine, cyclophosphamide, and rituximab, now used less often but still appropriate for selected patients
- Stem cell transplantation or cellular therapies for younger patients with high-risk or resistant disease
Supportive care matters too. People with CLL are more prone to infections, so vaccinations and prompt attention to fever are part of routine management. Our guide for patients and caregivers covers day-to-day living with blood disorders, and the leukemia guide compares CLL with other leukemias.
Key Takeaways
- The ICD 10 chronic lymphocytic leukemia code is C91.1; in ICD-10-CM use C91.10, C91.11, or C91.12 depending on documented status.
- SLL is coded separately under C83.0, even though it is biologically the same disease.
- Patients on watch and wait still have active disease and are coded C91.10.
- Diagnosis rests on a clonal B-cell count of at least 5 × 10^9/L with a typical flow cytometry pattern.
- Treatment is reserved for active disease and is increasingly based on targeted oral drugs.
Accurate coding reflects accurate clinical documentation, a theme that runs through all of hematology.
Frequently Asked Questions
Is C91.1 a billable code?
In ICD-10-CM, C91.1 is a category heading and is not billable on its own. You need the fifth character: C91.10, C91.11, or C91.12. Other countries using the WHO version of ICD-10 may record C91.1 directly.
Which code do I use for a patient on watch and wait?
Use C91.10, CLL not having achieved remission. The disease is present even though it is not being treated, so a history code would be incorrect.
How is CLL coded in ICD-11?
ICD-11 uses a different structure, and CLL/SLL falls under the mature B-cell neoplasm codes. Organizations still using ICD-10 or ICD-10-CM should continue with C91.1x until they formally transition.
Can CLL be cured?
For most people CLL is a chronic, controllable condition rather than a curable one. Modern treatments can produce long, deep remissions, and many patients live for many years with good quality of life.
When to See a Doctor
See a doctor if a blood test shows a persistently raised lymphocyte count, or if you notice painless swollen glands, drenching night sweats, unexplained weight loss, or repeated infections. People already diagnosed with CLL should report fever promptly, because infections can become serious quickly.
