Quick answer: The primary, or first-listed, diagnosis is the main reason for the visit or admission, as documented by the provider. Secondary diagnoses are other conditions that coexist and affect the patient’s care, treatment, or management during that encounter. Code to the highest specificity, use combination codes when ICD-10-CM provides them, and do not code conditions that no longer affect care.
How Do You Code Primary vs Secondary Diagnoses?
Knowing the difference between primary and secondary diagnoses helps you pick the right codes and supports correct payment. The primary diagnosis is the main reason for the visit. Secondary diagnoses are other conditions that are present but are not the main reason for the visit or admission.
This post covers the guidelines, best practices, and tips for coding both.
What Are Primary and Secondary Diagnoses?
Primary Diagnosis
- The primary diagnosis is the condition that is chiefly responsible for the patient’s reason for the encounter or hospitalization.
- This condition is the main reason the patient seeks medical attention, whether for diagnosis, treatment, or management.
- In the context of inpatient admissions, the principal diagnosis is the condition found after study to be chiefly responsible for the admission. For outpatient visits, it’s the condition that brought the patient to the healthcare provider.
Examples of primary diagnoses:
You may also find our article on Z codes for principal vs secondary diagnosis helpful.
A patient admitted for acute myocardial infarction (heart attack) (ICD-10 code: I21.9).
Similarly, a patient visiting for routine prenatal care (ICD-10 code: Z34.90).
A patient visiting for severe pneumonia (ICD-10 code: J18.9).
Secondary Diagnosis
- A secondary diagnosis refers to any condition that coexists with the primary diagnosis but is not the primary reason for the patient’s visit or admission.
- Secondary diagnoses may affect the treatment or outcome of the primary condition, but they are not the primary focus of care during the encounter.
- These conditions may be chronic diseases, comorbidities, complications, or other medical issues that contribute to the patient’s overall health status.
Examples of secondary diagnoses:
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- A patient with diabetes (ICD-10 code: E11.9) receiving treatment for a fracture (ICD-10 code: S52.501A, unspecified fracture of the lower end of right radius, initial encounter for closed fracture) after a fall. Category S52.5- needs more characters to be a complete code.
- A patient with hypertension (ICD-10 code: I10) hospitalized for acute stroke (ICD-10 code: I63.9).
- A patient with obesity (ICD-10 code: E66.9) and osteoarthritis (ICD-10 code: M19.90) visiting for routine care.
How Do You Decide Which Diagnosis Is Primary?
Follow Clinical Documentation
The provider should clinically document the primary diagnosis as the main reason for the encounter or hospitalization. Coders rely on the physician’s notes, admission records, and discharge summaries to determine which condition is primary.
For example, a patient may come in for surgery due to an injury but also have diabetes. In that case, the injury is the primary diagnosis. Diabetes is secondary because it affects the patient’s recovery or treatment, but it was not the reason for the admission.
Look for the Condition Requiring the Most Resources
A primary diagnosis often requires more intensive care or treatment resources. For example, a heart attack needs far more care than a chronic condition like hypertension or diabetes.
If the provider treats several conditions during the same visit or stay, list first the one that required most of the care.
Sequencing of Diagnoses
In ICD-10-CM, when coding multiple diagnoses, the primary diagnosis should always come first. Secondary diagnoses follow in order of significance, complexity, or how they impact the patient’s care.
Example: In a situation where a patient has pneumonia (primary diagnosis) and chronic obstructive pulmonary disease (COPD) (secondary diagnosis), list pneumonia first because it is the condition the provider treats in the encounter.
Sequencing Sequencing in Medical Coding 2026
Acute vs. Chronic Conditions
Coders usually list an acute condition first, even when the patient also has chronic conditions. For example, a patient with chronic heart failure (secondary diagnosis) might come in for acute heart failure (primary diagnosis).
Chronic conditions like diabetes or hypertension are usually secondary unless the provider actively manages or treats them during the visit.
Chronic Medical Coding for Chronic Conditions
Comorbidities and Complications
A comorbidity (a pre-existing condition) or a complication (a condition that arises during treatment) may be a secondary diagnosis. For example, if a patient with type 2 diabetes is admitted for a diabetic foot ulcer, list the combination code E11.621 (Type 2 diabetes mellitus with foot ulcer) first. Then add a code from L97.- for the site and severity of the ulcer.
If a patient develops sepsis due to a postprocedural infection, sequence the postprocedural infection codes from T81.4- first, including T81.44 (sepsis following a procedure). Then add the sepsis code, such as A41.9. If the provider documents severe sepsis, also add a code from R65.2-.
Co-existing Medical Coding for Co-Existing Conditions
What Are Some Tips for Coding Primary and Secondary Diagnoses?
Ensure Accurate Documentation
Thorough documentation by the healthcare provider is key to assigning the correct primary and secondary diagnoses. Providers should document the main reason for the visit and any other conditions that affect the patient’s health.
Coders should be familiar with clinical terminology and understand how each diagnosis impacts the patient’s treatment to avoid misclassification.
Use ICD-10-CM Codes Effectively
ICD-10-CM is more specific than ICD-9-CM, so always choose the most specific code available.
For secondary diagnoses, make sure the record clearly shows how the condition affects the patient’s care. For example, if a patient with asthma receives treatment for acute respiratory distress, the provider should document asthma as a secondary condition.
Review Guidelines for Hierarchical Conditions
Risk adjustment models group conditions into hierarchies, so the most severe condition in a group drives the risk score. For example, diabetes with complications outranks diabetes without complications in the HCC model. However, HCC hierarchies do not change ICD-10-CM sequencing, so always follow the official guidelines when you sequence.
Understand Payer Policies
Different insurers may have varying guidelines regarding the use of primary and secondary diagnoses. Some payers may prioritize certain conditions over others when determining reimbursement.
For example, Medicare Advantage plans use Hierarchical Condition Categories (HCC) to set risk scores, so accurate reporting of comorbidities can change payment a great deal.
Follow Coding Conventions
Follow coding conventions that dictate when to consider a diagnosis primary. For example, the ICD-10-CM includes guidelines on how to code multiple diagnoses when they are related to specific body systems (e.g., coding for diabetes with nephropathy).
Pay close attention to combination codes that report two conditions in one code, such as E11.21 for type 2 diabetes with diabetic nephropathy or E11.22 for type 2 diabetes with diabetic chronic kidney disease.
Avoid Overcoding or Undercoding
Do not overcode by reporting conditions that do not affect the current visit or treatment. On the flip side, undercoding can lead to under-reimbursement and inaccuracies in medical records.
Code only conditions that affect care now. Skip resolved or historical conditions unless they still affect care.
Bundling and Upcoding Medical Coding Bundling and Upcoding
Properly differentiating between primary and secondary diagnoses is critical for accurate coding, compliance, and reimbursement. Accurate provider documentation and precise coding also make sure the record captures every diagnosis. When coders know these rules, they prevent denials, support correct payment, and show the true complexity of care.
These tips make coding and billing more accurate and efficient for providers, patients, and payers.