9.1 Infectious Disease, Neoplasms, Blood, and Endocrine

Infectious Disease, Neoplasms, Blood, and Endocrine

Quick Answer: For the 15-question AAPC Certified Outpatient Coder (COC) ICD-10-CM domain, Chapters A00–B99, C00–D49, D50–D89, and E00–E89 are less about memorizing every subcategory and more about chapter-specific traps. First-list the reason for this encounter (FY 2026 Official Guidelines Section IV). Then apply Section I.C.1–4: confirmed HIV (B20 versus Z21), high-level sepsis and resistant-organism rules, the Neoplasm Table columns (primary, secondary, in situ, history, antineoplastic encounter), anemia in neoplastic disease sequencing, and diabetes mellitus combination codes such as E11.9 versus E11.65 versus hyperosmolar and ketoacidosis subcategories.

This independent OpenExamPrep section helps learners study International Classification of Diseases, Tenth Revision, Clinical Modification (ICD-10-CM) medical-chapter diagnosis rules for hospital outpatient and ambulatory surgery center (ASC) facility coding. It is not a Centers for Medicare & Medicaid Services (CMS), National Center for Health Statistics (NCHS), or American Academy of Professional Coders (AAPC) product, and it does not claim partnership or official review by those organizations. Open the FY 2026 Official Guidelines PDF and the Tabular List; do not treat this page as a substitute for either book.

Why these four chapters show up on facility claims

Same-day surgery, hospital outpatient infusion, and observation stays constantly carry infectious status, a current or historical malignancy, anemia, and diabetes. The Current Procedural Terminology (CPT) code explains the resource; the ICD-10-CM code explains why this visit happened. If you first-list a stable chronic diabetes code when the patient came only for a screening colonoscopy, or if you first-list a personal-history Z code while chemotherapy is still directed at a current primary, you have missed Section IV and Section I.C.2 in the same stroke.

Chapter-specific guidelines apply in all settings unless a later section says otherwise. What changes in the outpatient hospital and ASC is the first-listed test: the condition chiefly responsible for this encounter, after conventions and disease-specific notes. Do not code “probable,” “possible,” or “rule out” infection, cancer, or diabetes as if confirmed. Code to the highest certainty documented for that visit.

Infectious disease — HIV, organisms, resistance, and sepsis (high-level)

Code only confirmed HIV. The provider’s diagnostic statement that the patient is HIV positive or has an HIV-related illness is enough; a printed lab report is not required. That confirmation rule is an exception to inpatient uncertain-diagnosis logic, and it still matters in the outpatient setting because you never invent B20 from a “rule out HIV” note.

Once an HIV-related illness has developed, B20 stays. Antiretroviral medication does not, by itself, convert Z21 into B20 or B20 into Z21; the documented disease versus status does.

Infectious agents classified elsewhere. When an infection lives in another chapter and the infection code does not name the organism, an additional code from B95–B97 identifies the agent if the Tabular List asks for it. Do not add a Chapter 1 organism code that duplicates a combination code that already includes the organism.

Infections resistant to antibiotics. Identify documented antibiotic resistance. Assign a code from category Z16, Resistance to antimicrobial drugs, after the infection code only if that infection code does not already identify the resistance.

Methicillin-resistant Staphylococcus aureus (MRSA) is the classic trap. Sepsis due to MRSA is A41.02. Pneumonia due to MRSA is J15.212. Those combination codes already include the organism and the resistance. Do not add B95.62 or Z16.11 (resistance to penicillins) on top of them. When the infection has no combination organism code (for example, a wound infection due to MRSA), assign the site infection plus B95.62, still without Z16.11. Colonization or carriage without illness is Z22.322 (MRSA carrier), which is not the same as current MRSA infection.

Sepsis, severe sepsis, and septic shock are less common on an ASC claim than in the emergency department or observation, but the guidelines still apply when the condition is documented. For sepsis, assign the underlying systemic infection code; if the organism is not specified, A41.9, Sepsis, unspecified organism. Do not assign R65.2, Severe sepsis, unless severe sepsis or associated acute organ dysfunction is documented. R65.2 is never first-listed. Septic shock is sequenced infection first, then R65.21 (or T81.12- for postprocedural septic shock). Urosepsis is not a synonym for sepsis; it has no default Index code — query. Negative blood cultures do not by themselves rule sepsis in or out; query if the diagnostic statement and the cultures conflict. If the reason for the encounter is sepsis plus a localized infection such as pneumonia or cellulitis, the systemic infection is first and the localized infection is additional — unless a more specific chapter note (for example, HIV, obstetrics) takes sequencing priority.

Keep sepsis teaching at this high level on the COC: identify the systemic infection, do not first-list R65.2, do not treat urosepsis as A41.9, and do not stack Z16 onto a combination MRSA code.

Neoplasms — table columns, current versus history, antineoplastic encounters

Chapter 2 holds most benign and all malignant neoplasms. Some benign lesions (the guidelines name prostatic adenomas as the pattern) live in body-system chapters. Coding starts with the record: benign, in situ, malignant, or uncertain behavior. If malignant, find any secondary sites. If a histologic term is documented, look up that term in the Alphabetic Index first so you know which Neoplasm Table column to use; “adenoma,” for example, points you toward the benign column rather than a blind jump into the table. Then verify the code in the Tabular List. Do not paste or memorize the printed table; learn the columns and the sequencing notes.

Anemia associated with malignancy (treatment is only the anemia): sequence the malignancy first, then D63.0, Anemia in neoplastic disease. That is an explicit exception to the usual “complication first when only the complication is treated” neoplasm rule.

Anemia associated with chemotherapy, immunotherapy, or radiotherapy (treatment is only the anemia): sequence the anemia first (for example D64.81, Anemia due to antineoplastic chemotherapy), then the neoplasm, then the adverse-effect code (T45.1X5- for antineoplastic drugs; Y84.2 when the anemia is an abnormal reaction to radiotherapy).

Dehydration treated alone is sequenced dehydration first, then the malignancy. A surgical complication of neoplasm treatment is first-listed as the complication when treatment is directed at resolving it. A pathologic fracture in neoplastic disease (M84.5-) is first-listed when the encounter is for the fracture; if the encounter is for the neoplasm and the fracture is associated, the neoplasm is first.

Symptoms in Chapter 18 that are characteristic of the malignancy do not replace the neoplasm as first-listed. C80.0 (disseminated malignant neoplasm, unspecified) is only for advanced metastatic disease with no known primary or secondary sites specified. C80.1 (malignant primary neoplasm, unspecified) is cancer unspecified and should be rare when a site is knowable.

Blood and blood-forming organs (D50–D89)

Chapter 3 currently has no expanded chapter-specific guideline beyond “reserved for future expansion.” Facility coders still assign anemia, coagulation, and immune-mechanism codes at the highest documented specificity, and they still obey the Chapter 2 anemia-sequencing notes above. D63.0 is the manifestation-style anemia of neoplastic disease; it is not a substitute for iron-deficiency D50.9 or for chemotherapy-induced D64.81 when the record names those causes instead.

Endocrine — diabetes combination codes, use-additional notes, hyperosmolarity versus ketoacidosis

Diabetes mellitus codes in E08–E13 are combination codes: they include the type, the body system affected, and the complication. Assign as many codes from E08–E13 as needed to describe all associated conditions. Sequence them from the reason for this encounter. Age alone does not decide type 1 versus type 2.

If the type is not documented, the default is E11.- Type 2 diabetes mellitus. If the type is not documented but insulin is used, still default to E11.- and add long-term-use codes from Z79 as applicable. Z79.4 (long-term current use of insulin) is not assigned for insulin given only temporarily to control glucose during this encounter. If the patient uses both insulin and an oral hypoglycemic, assign Z79.4 and Z79.84. If insulin and an injectable non-insulin antidiabetic are both long-term, assign Z79.4 and Z79.85.

E11.9, Type 2 diabetes mellitus without complications, is correct only when no diabetic complication is documented. E11.65, Type 2 diabetes mellitus with hyperglycemia, is the combination code when hyperglycemia is documented without a more specific metabolic crisis. Hyperglycemia is not automatically ketoacidosis and is not automatically hyperosmolarity.

Use additional notes still apply. E11.22, Type 2 diabetes mellitus with diabetic chronic kidney disease, requires an additional N18.- code for the chronic kidney disease stage. Diabetic retinopathy, neuropathy, and circulatory complication codes are assigned when documented; you may need more than one E11 code at the same visit. FY 2026 also provides E11.A, Type 2 diabetes mellitus without complications in remission, only when the provider documents remission (not a casual “resolved”), and E10.A- for presymptomatic type 1 diabetes. Secondary diabetes (E08, E09, E13) follows Tabular “code first” notes for the underlying cause; postpancreatectomy diabetes uses E13.- with E89.1 and Z90.41 as the guidelines instruct.

Facility scenario

Hospital outpatient infusion: current right-breast primary still on a chemotherapy protocol; today’s orders are antineoplastic infusion only. First-list Z51.11, then the current breast malignancy (not Z85.3). Two days later the same patient returns only for transfusion of anemia the provider links to the malignancy, with no antineoplastic drug given. First-list the malignancy, then D63.0. If instead the anemia is documented as an adverse effect of the chemotherapy and only the anemia is treated, the anemia code is first, then the neoplasm, then T45.1X5-. On a later ASC day for laparoscopic cholecystectomy, type 2 diabetes with hyperglycemia and long-term insulin is additional, not first-listed, unless the diabetes is actually why the patient is there: report E11.65 and Z79.4, not E11.9, and not E11.10 without ketoacidosis.

Source

FY 2026 ICD-10-CM Official Guidelines for Coding and Reporting (CMS PDF) — Section I.C.1–4 and Section IV.