ICD-10-CM Official Guidelines (FY2027): A Study Guide for Coding Students

1. Signs and Symptoms vs. Confirmed Diagnoses

A sign or symptom code is appropriate when the provider has not yet established a related definitive diagnosis. Many symptom codes are in Chapter 18 (R00-R99), but some are in the body-system chapters. Once the provider confirms a diagnosis, code the diagnosis rather than the symptoms it explains.

Code each encounter to the level of certainty known at that encounter. Unspecified codes and symptom codes are legitimate, even necessary, when that is all the record supports. Do not choose a more specific code than the documentation supports. A condition the provider calls 'borderline' at discharge is coded as confirmed, unless the Index has a specific entry for the borderline form, such as borderline diabetes.

A code is assigned on the provider's statement that the condition exists. You do not apply your own clinical criteria. If the documentation conflicts, query the provider.

2. Conditions Integral to a Disease Process

Signs and symptoms that routinely come with a disease are part of that disease. Do not code them separately once the disease is coded, unless the classification instructs you to. For example, do not add a code for the typical pain of a condition whose definition includes that pain.

Signs and symptoms that are not routinely associated with the disease should be coded when present. When you report a symptom together with its related definitive diagnosis, sequence the definitive diagnosis first. Some codes are combination codes that already include a common symptom; when you use one, do not add a separate code for that symptom.

3. Sequela (Late Effects)

A sequela is a residual condition that remains after the acute phase of an illness or injury has ended. Examples include a scar after a burn, a deviated septum after a nasal fracture, or a neurologic deficit after a stroke. There is no time limit: a sequela can appear early or years later.

Sequela coding usually takes two codes. List the residual condition (the nature of the sequela) first. List the code showing what caused it (the sequela code) second. The exception is when the sequela code itself includes the manifestation, or is followed by a manifestation code in its title; then the single code, or the specified pair, is used. Never report the code for the acute phase of an illness or injury together with a code for its late effect.

In Chapter 19, a sequela is shown by adding 7th character S to the original injury code. The S goes only on the injury code, not on the code for the residual condition, and the residual condition is listed first.

4. Impending or Threatened Conditions

When the provider describes a condition at discharge as 'impending' or 'threatened,' first ask whether it actually happened. If it did, code it as a confirmed diagnosis.

If it did not happen, look in the Alphabetic Index for a subterm 'impending' or 'threatened' under the condition. Also check the main terms 'Impending' and 'Threatened.' If an entry exists, assign that code. If no entry exists, code the underlying condition or conditions that are present, not the condition that was only threatened.

5. Sepsis, Severe Sepsis, and Septic Shock

For sepsis, assign the code for the underlying systemic infection. If the organism is not specified, use A41.9 (Sepsis, unspecified organism). The term 'urosepsis' is not the same as sepsis and has no default code, so query the provider. Negative or inconclusive blood cultures do not rule out sepsis, but the provider should be queried.

Severe sepsis is sepsis with associated acute organ dysfunction. It needs at least two codes: the systemic infection first, then a code from R65.2-, plus codes for each organ dysfunction. Use R65.20 when there is no septic shock and R65.21 when septic shock is present. Do not assign R65.2- unless the provider documents severe sepsis or an organ dysfunction associated with the sepsis. If the record ties the organ dysfunction to a different condition, do not assign R65.2-. If the link is unclear, query. A code from R65.2- can never be the principal diagnosis.

Sequencing depends on timing. If sepsis and a localized infection such as pneumonia are both present on admission, sequence the systemic infection first and the localized infection as a secondary diagnosis. If the patient is admitted with the localized infection and sepsis develops later, sequence the localized infection first. Sepsis due to a postprocedural infection starts with the complication code; for a surgical site infection, that is a code identifying the site (T81.41- through T81.43-, or T81.49-), followed by T81.44- for sepsis following a procedure.

6. Neoplasms: Primary, Secondary, and History Of

If the documentation names a histology (for example, adenoma), look up that term in the Index first. It tells you which column of the Neoplasm Table to use: malignant primary, malignant secondary, in situ, benign, uncertain behavior, or unspecified behavior. Then verify the code in the Tabular List.

Sequencing depends on what the encounter treats. Treatment directed at the primary site: the primary malignancy is listed first, followed by any metastatic sites. Treatment directed only at a metastatic site: the secondary site is listed first, even though the primary is still present. Encounter chiefly for chemotherapy, immunotherapy, or external beam radiation: Z51.11, Z51.12, or Z51.0 is listed first and the cancer second. Encounter to manage anemia associated with the malignancy: the malignancy is listed first, followed by D63.0. For other complications treated alone, such as dehydration, the complication is listed first.

Use a Z85 personal history code only when the primary malignancy has been excised or eradicated, no treatment is directed to that site, and there is no evidence of disease there. While treatment such as chemotherapy or radiation is still directed at the site, keep coding the current malignancy. If cancer has spread from a previously removed primary, code the secondary site and add the Z85 code for the former primary site.

7. Diabetes Mellitus and Its Complications

Diabetes codes are combination codes. Each one captures the type of diabetes, the body system affected, and the complication. Assign as many codes from the same category as needed to describe every complication, and sequence them by the reason for the encounter. The word 'with' in the Index presumes a cause-and-effect link. A condition listed under 'with' beneath diabetes is coded as a diabetic complication unless the provider states it is not related.

If the type is not documented, the default is type 2 (E11.-). Type 1 is not determined by age alone. Type 2 diabetes in remission (E11.A) requires the provider to document remission; 'resolved' does not mean remission. Add Z79 codes for long-term use of insulin (Z79.4), oral hypoglycemics (Z79.84), or injectable non-insulin antidiabetic drugs (Z79.85). If a patient uses more than one type, assign a code for each. Do not assign Z79.4 when insulin is given only temporarily to bring blood sugar under control.

Secondary diabetes (E08, E09, E13) is caused by another condition or event. Follow the Tabular List instructions for sequencing. For diabetes after a pancreatectomy, assign E89.1, a code from E13, and Z90.41-. In pregnancy, an O24 code is listed first. Gestational diabetes (O24.4-) is never coded with other O24 codes or with the Z79 diabetes drug codes.

8. Hypertension with Heart or Kidney Disease

The classification presumes a cause-and-effect link between hypertension and certain heart conditions, and between hypertension and chronic kidney disease (CKD). Code them as related unless the provider clearly states they are unrelated.

Hypertension with heart failure (I50.-) or certain other heart conditions (I51.4, I51.89, I51.9): use I11. Add the I50 or I51 code for the specific condition. With I51.5 or I51.7, I11 alone is enough. Hypertension with CKD: use I12, plus an N18 code for the stage. Hypertension with both heart disease and CKD: use I13, plus N18 for the stage and I50 for the type of heart failure. Do not report I11 or I12 when I13 applies. Other conditions, such as coronary artery disease, are not presumed to be linked; the provider must document the connection.

Related rules: For hypertensive cerebrovascular disease, list the I60-I69 code first, then the hypertension code. For secondary hypertension, assign two codes: one for the underlying cause and one from I15. 'Controlled' and 'uncontrolled' hypertension are still coded from I10-I15. An elevated reading without a hypertension diagnosis is R03.0. Hypertensive urgency or emergency goes to I16, coded with any identified hypertensive disease. Resistant hypertension (I1A.0) is an additional code, listed after the code for the type of hypertension.

9. Acute Myocardial Infarction (AMI) Timing

A myocardial infarction (MI) is considered acute for 4 weeks. While the MI is 4 weeks old or less, I21 codes may still be reported, including after a transfer to another acute or post-acute setting, as long as the MI meets the definition of an additional diagnosis. After 4 weeks, if care related to the MI continues, assign the appropriate aftercare code instead of I21. For an old or healed MI that needs no further care, use I25.2.

A new type 1 or unspecified MI within 4 weeks of a type 1 or unspecified MI is coded with I22 together with I21. Sequence them by the circumstances of the encounter. Do not use I22 for other types. A subsequent type 2 MI is coded only as I21.A1. A subsequent type 4 or type 5 MI is coded only as I21.A9. If the two MIs are different types, assign an I21 code for each.

Type rules: A type 1 NSTEMI that evolves to a STEMI is coded as STEMI. A STEMI that converts to NSTEMI after thrombolytic therapy is still coded as STEMI. A type 2 MI (demand ischemia) is I21.A1, with the underlying cause listed first if applicable, even when it is described as NSTEMI or STEMI. An MI with no type or site specified defaults to I21.9. A type 1 STEMI with no site documented is I21.3. If a patient with coronary artery disease is admitted for an AMI, list the AMI first.

10. COPD and Asthma

The codes for chronic obstructive pulmonary disease (COPD, category J44) and asthma (category J45) separate uncomplicated cases from cases in acute exacerbation. An acute exacerbation is a worsening, or decompensation, of the chronic condition.

An exacerbation is not the same as an infection on top of the chronic disease, although an infection can trigger one. Do not assign an exacerbation code just because an infection is present; the provider must describe a worsening of the chronic condition. When both an exacerbation and an infection are documented, code the infection as well, following the Tabular List instructions.

11. Injuries: 7th Characters and Multiple Injuries

Most Chapter 19 codes require a 7th character. A (initial encounter) is used for every encounter in which the patient receives active treatment, even if a different provider is seeing the patient. D (subsequent encounter) is used after active treatment is finished, while the patient receives routine care during healing or recovery. S (sequela) identifies the injury that caused a late effect. Fractures have additional 7th characters for open fractures and for nonunion and malunion. Do not use aftercare Z codes for injuries; assign the injury code with the subsequent-encounter 7th character instead. If a code has fewer than 6 characters, fill the empty positions with the placeholder X so the 7th character is in the 7th position.

Code each injury separately unless a combination code exists. List the most serious injury first, as determined by the provider and the focus of treatment. Do not code superficial injuries such as abrasions when there is a more severe injury of the same site. Injuries caused by medical care are coded as complications, not with S or T injury codes.

Fracture defaults: a fracture not documented as open or closed is coded as closed. A fracture not documented as displaced or nondisplaced is coded as displaced. An open fracture with no Gustilo type documented gets the 7th character for open type I or II. For a patient with known osteoporosis who breaks a bone in a minor fall that would not break a normal, healthy bone, use M80, not a traumatic fracture code.

12. Burns: Depth and Extent

Burn codes cover thermal burns from heat sources such as fire or hot appliances, plus burns from electricity and radiation. Sunburn is excluded. Corrosions are chemical burns and follow the same guidelines. Burns of the external body (T20-T25) are classified by depth: first degree (erythema), second degree (blistering), and third degree (full-thickness involvement). Burns of the eye and internal organs (T26-T28) are classified by site, not degree.

Assign a separate code for each burn site, and list the highest-degree burn first. If one site on the same side has burns of different degrees, code only the highest degree. Burns that are not healing are coded as acute burns, and necrosis of burned skin is coded as a non-healed burn. For an infected burn, add a code for the infection. Use an external cause code to show the source of the burn, the intent, and the place where it occurred.

Extent of body surface is coded with T31 (burns) or T32 (corrosions). Use these when the burn site is not specified or when additional data is needed. A T31 code is advisable when a third-degree burn covers 20 percent or more of the body surface. The rule of nines estimates the percentage: head and neck 9, each arm 9, each leg 18, anterior trunk 18, posterior trunk 18, genitalia 1. Do not use T31 or T32 for burn sequelae. Late effects such as scars or contractures are coded with the burn code plus 7th character S.

13. Poisoning vs. Adverse Effect vs. Underdosing

Adverse effect: the right drug was correctly prescribed and properly given, but the patient had a reaction. Code the reaction first (for example, vomiting or tachycardia), then the T36-T50 code with 5th or 6th character 5. Poisoning: the drug was used improperly. Examples are an overdose, a wrong drug, a wrong route, a prescribing or dosing error, a nonprescribed drug taken with a prescribed one, or a drug taken with alcohol. List the T36-T50 poisoning code first, then the manifestations. If the intent is unknown, code it as accidental. Use 'undetermined' only when the record states the intent cannot be determined.

Underdosing means the patient took less medication than prescribed, or stopped a prescribed drug on their own. Use the T36-T50 code with 5th or 6th character 6. Documentation that the patient took less than prescribed is enough; a change in condition is not required. An underdosing code is never listed first. If the underlying condition relapsed or worsened, code that condition, and add a noncompliance code (Z91.12-, Z91.13-, Z91.14-, Z91.A4-) or a complication-of-care code (Y63.6-Y63.9) to show intent.

Toxic effects of nonmedicinal substances (T51-T65) are coded first, followed by their manifestations. Never code directly from the Table of Drugs and Chemicals; always verify in the Tabular List. T36-T65 codes already include the intent, so no external cause code is needed to show it.

14. Pregnancy: First-Listed Chapter 15 Codes

Chapter 15 codes (O00-O9A) take sequencing priority over codes from other chapters and are used only on the mother's record, never the newborn's. Codes from other chapters may be added for more detail. If the provider documents that the pregnancy is incidental to the visit, use Z33.1 instead of Chapter 15 codes. Assign the trimester character based on the provider's documentation of trimester or weeks of gestation. Use an unspecified trimester only when the trimester cannot be determined and clarification cannot be obtained.

First-listed codes by situation: For a routine prenatal visit with no complications, use Z34.-, which is never combined with Chapter 15 codes. For a routine prenatal visit for a high-risk pregnancy, use O09.-. For an episode with no delivery, list the main complication that prompted the encounter. For a delivery admission, list the condition that prompted the admission. In a cesarean delivery, that is the condition that led to the cesarean, if the patient was admitted with it. Add a Z37 outcome-of-delivery code to every delivery record.

O80 is used only for a full-term, uncomplicated delivery of a single healthy infant. It is always the principal diagnosis and is used only with Z37.0. Pre-existing diabetes in pregnancy is coded with O24 first, then the E08-E13 code. HIV disease in pregnancy is coded O98.7- followed by B20, or by Z21 if asymptomatic. Z3A codes may be added to show weeks of gestation, but not for abortive outcomes, elective terminations, or postpartum conditions.

15. Z Codes: Screening, History, Status, Aftercare, Follow-up

Screening: testing a person who appears well, in order to detect disease early. If the patient has a sign or symptom, the test is diagnostic, not screening, so code the sign or symptom. A screening code can be first-listed when screening is the purpose of the visit. A condition found during screening is coded as an additional diagnosis. History: a personal history code (Z85-Z87, among others) means the condition no longer exists and is not being treated. A family history code means a relative had the disease. Status: a status code means the patient carries a disease or has a residual of past care, such as a device or transplant. Do not use a status code when the diagnosis code already contains that information; for example, do not report Z94.1 with a T86.2- code.

Aftercare: the initial treatment is done, and the patient needs continued care during healing or recovery. Do not use aftercare codes for a current, acute disease or for injuries. Follow-up (Z08 and Z09): surveillance after treatment is complete and the condition no longer exists. List the follow-up code first, then the history code. If the condition has recurred, code the condition instead of the follow-up code.

16. Outpatient (Section IV): Uncertain Diagnoses, Chronic Conditions, Pre-op Evaluations

Uncertain diagnoses: In outpatient settings and physician offices, do not code a diagnosis described as probable, suspected, questionable, rule out, compatible with, consistent with, or working diagnosis. Code to the highest degree of certainty, such as symptoms, signs, or abnormal test results. This is the opposite of the rule for inpatient admissions to short-term acute, long-term care, and psychiatric hospitals, where an uncertain diagnosis at discharge is coded as if it were confirmed. Some conditions are coded only when confirmed, in every setting: HIV, COVID-19, Zika, and influenza due to certain identified viruses.

Chronic diseases: a chronic disease treated on an ongoing basis may be coded at every visit where it is treated. Code all documented coexisting conditions that require or affect care, treatment, or management. Do not code conditions that were treated before and no longer exist, although history codes may be used if they affect current care. For an outpatient diagnostic test that a physician has interpreted, with the final report available at the time of coding, code the confirmed diagnosis and not the related symptoms.

Pre-operative evaluations: when the encounter is only for pre-op clearance, list a Z01.81- code first. Add the condition that is the reason for the surgery, and any findings from the evaluation.

17. First-Listed Diagnosis in Outpatient Settings

Outpatient records use the term 'first-listed diagnosis' instead of 'principal diagnosis,' and the inpatient (UHDDS) definition of principal diagnosis does not apply. List first the diagnosis, condition, problem, or other reason chiefly responsible for the services provided. This may be a symptom if no diagnosis has been confirmed. Coding conventions and the general and chapter-specific guidelines take precedence over the outpatient guidelines. Always begin the search in the Alphabetic Index, not the Tabular List.

Specific situations: Outpatient surgery: list the reason for the surgery first, even if the surgery was cancelled because of a contraindication. Ambulatory surgery: if the postoperative diagnosis differs from the preoperative one and is confirmed when you code, use the postoperative diagnosis because it is more definitive. Observation for a medical condition: list that condition first. Surgery followed by observation for a complication: list the reason for the surgery first, then the complication. Encounter chiefly for chemotherapy, immunotherapy, or radiation therapy: list the Z51 code first. Routine exam that finds something new or worse: use the general exam code 'with abnormal findings' first, and add a code for the finding.