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Practice Fusion · Oct 19, 2021

The 25 most common medical diagnoses

If you were practicing medicine in October 2015, you undoubtedly remember the tremendous change associated with the transition from ICD-9 to ICD-10. All HIPAA-covered entities, including healthcare providers, healthcare clearing houses, electronic health record (EHR) vendors, and other business associates, were required to start using ICD-10 for medical billing and clinical documentation. ICD-10 is the tenth revision of the International Statistical Classification of Diseases and Related Health Problems (ICD) from the World Health Organization (WHO). Healthcare providers from all over the world were already using ICD-10 when those from the United States were required to do so.

In this article, we explore some of the history behind the switch from ICD-9 to ICD-10 and share the 25 most common medical diagnoses in 2021.

Why was the change from ICD-9 to ICD-10 necessary?

As you know, ICD is a diagnostic tool used for clinical, health management, and epidemiologic purposes, enabling analysis, assessment, and comparison of the incidence and prevalence of medical conditions, morbidity, and mortality data across the world. ICD-10 includes updated codes for signs and symptoms, diseases, procedures, abnormal findings, external causes of injury or disease, and social circumstances. Importantly, ICD-10 is much more than a simple updated version of ICD-9. Rather, it’s essentially a complete revamp and modernization of medical classifications, including nearly 70,000 new codes and more detailed 7-digit alphanumeric codes.1,2,3

ICD-10 uses current medical terminology via an expanded coding format, enabling healthcare providers to capture greater detail within the code, providing much more specific information about the diagnosis. It is much more flexible than ICD-9, reflecting medical advances, new technologies, and diagnoses, while including more information about ambulatory and managed care as well as accidents and injuries. ICD-10 allows much greater specificity, including the ability to add laterality, combine diagnosis and symptom codes, and improve identification of conditions and disease processes to further promote research.1,2,3

Further, this transition was required, by law, for covered entities—such as physicians, other healthcare providers, EHR vendors, and more–under the Health Insurance Portability and Accountability Act (HIPAA). It’s also mandatory for healthcare providers to bill using ICD-10 codes to receive payment for their services; otherwise, payment will be denied. Technically, the HIPAA ICD-10 code requirements apply solely to electronic transactions. Yet using ICD-10 for electronic submissions and ICD-9 for manual submissions would be extremely burdensome regarding time, duplicative efforts, error risk, resources, and other costs.1,2,3

What are the major differences between ICD-10 and ICD-9?

  • ICD-9 codes are 3 to 5 characters, whereas ICD-10 codes range from 3 to 7 characters.
  • ICD-9 codes usually use just numbers, yet ICD-10 codes comprise both letters and numbers.
  • There are approximately 13,000 ICD-9 codes, yet ICD-10 contains nearly 70,000 codes
  • Many ICD-9 codes use obsolete, outdated codes that do not reflect current medical practices.
  • ICD-10 offers much more flexibility for adding new codes, whereas the structure of ICD-9 codes limited the number of new codes that could be established. In fact, many ICD-9 categories are full.
  • ICD-10 codes capture much more detail than ICD-9, including the new ability to add laterality.

Assessing your patient populations by ICD-10 for quality of care

To ensure optimal quality and managed care for your patients, it’s helpful to gain insight into trends in ambulatory care in the United States and in your area of the country. Identifying and tracking specific sets of patient populations is also key for fulfilling quality improvement measures as part of your practice’s Merit-based Incentive Payment Systems (MIPS) score.

Practice Fusion enables you to create custom reports of your practice’s patient populations based on using different combinations of clinical and demographic criteria along with the ability to save your customized reports for repeated generation. Practice Fusion’s Patient List Report provides several search parameters, including:

  • Age range
  • One or more diagnoses
  • Encounter type and date range
  • One or more procedures and date range
  • One or more medications by RxNorm code, enabling you to capture patients who are on either brand name or generic medications and ensuring that all applicable patients are included in your search results
  • Lab test results for patients with specific final lab results by LOINC code (an industry standard format) and result date or date range
  • Patient risk score, range of scores, or minimum and maximum score

What are the top 25 ICD-10 codes in 2021?

It’s important to identify the kinds of conditions most encountered in ambulatory practices, particularly those specializing in primary care, enabling providers to determine the context for where they should focus disease interventions.

The most common ICD-10 diagnoses seen by U.S. primary care specialists include essential (primary) hypertension (I10); type II diabetes mellitus without complications (E11.9) and other specified diabetes mellitus without complications (E13.9); and hyperlipidemia, unspecified (E78.5).4,5

It’s illustrative to compare the previous ICD-9 descriptions and codes against the new ICD-10 describers and codes or these frequent diagnoses regarding specificity and updated medical terminology:

  • unspecified essential hypertension (401.9)
  • diabetes mellitus without mention of complications, type II or unspecified type, not stated as uncontrolled (250.00)
  • other and unspecified hyperlipidemia (272.4)

The following chart shows 25 of the most common ICD-10 diagnoses as submitted by primary care specialists as well as the original ICD-9 codes.4,5 Regarding numerical rank, these are not in any particular order, since different sources may list slightly different numerical rankings.

However, overall, the same diagnoses do tend to rise to the top. For example, some primary care practitioners may report more cases of essential hypertension than type II diabetes mellitus, whereas for others, the reverse may be true. Yet importantly, the following currently represents the most frequently reported ICD-10 codes and shows clear trends regarding primary cares’ patient populations and where disease interventions and proactive prevention methods should be focused.

ICD-9 ICD-9 Description ICD-10 ICD-10 Description


unspecified essential hypertension


essential (primary) hypertension


diabetes mellitus without mention of complications, type II or unspecified type, not stated as uncontrolled



type II diabetes mellitus without complications

other specified diabetes mellitus without complications


other and unspecified hyperlipidemia


hyperlipidemia, unspecified

(*There are more specific ICD-10 codes available, if applicable, e.g., pure hypercholesterolemia [E78.0].)




low back pain


routine general medical examination at a healthcare facility


encounter for general adult medical examination without abnormal findings


chronic airway obstruction, not elsewhere classified


chronic obstructive pulmonary disease (COPD), unspecified

(*There are more specific ICD-10 codes, if applicable, e.g., chronic obstructive pulmonary disease with acute lower respiratory infection [J44.0]; an additional code is required to identify infection type.)


atrial fibrillation





Paroxysmal atrial fibrillation

Persistent atrial fibrillation

Chronic atrial fibrillation

Unspecified atrial fibrillation


abdominal pain, unspecified site


abdominal pain, unspecified

(*There are more specific ICD-10 codes, if applicable, e.g., acute abdominal pain [R10.0)], upper abdominal pain, unspecified [R10.10], right upper quadrant pain [R10.11], left upper quadrant pain [R10.12])


coronary artery atherosclerosis of unspecified type of vessel, native, or graft


*no equivalent in ICD-10; more clinical specifics are required to determine proper ICD-10 code. (However, one of the most frequent ICD-10 codes used by primary care specialists is “atherosclerotic heart disease of native coronary artery without angina pectoris” [I25.10].)


need for prophylactic vaccination and inoculation against unspecified single disease


encounter for immunization (ICD-10 code required first for any routine childhood examination.)


urinary tract infection, site not specified


urinary tract infection, site not specified (*Required to use an additional ICD-10 code to specify cause of infection [B95-97].) (*There are also more specific ICD-10 codes available, if applicable, e.g., acute cystitis without hematuria [N30.00], acute cystitis with hematuria [N30.01], interstitial cystitis chronic without hematuria [N30.10].)


anxiety state, unspecified


anxiety disorder, unspecified (*There are more specific ICD-10 codes, if applicable, e.g., generalized anxiety disorder [F41.1].)


depressive disorder, not elsewhere classified


major depressive disorder, single episode, unspecified (*There are more specific ICD-10 codes, if applicable, e.g., major depressive disorder, single episode, mild [F32.0], major depressive disorder, single episode, moderate [F32.1], etc.)


esophageal reflux



gastroesophageal reflux disease without esophagitis

gastroesophageal reflux with esophagitis


pain in limb


pain in unspecified limb (*There are more specific ICD-10 codes, if applicable, e.g., pain in right arm [M79.601], pain in left arm [M79.602], pain in arm, unspecified [M79.603], pain in right leg [M79.604], pain in left leg [M79.605], pain in leg, unspecified [M79.606], etc.


chest pain, unspecified


chest pain, unspecified (*There are more specific ICD-10 codes available, if applicable, e.g., chest pain on breathing [R07.1] or pleurodynia [R07.81].)


other malaise and fatigue






neoplasm (malignant) related fatigue (*Required to first code the associated neoplasm.)


other malaise

other fatigue

postviral fatigue syndrome


acute upper respiratory infections of unknown site



acute upper respiratory infection, unspecified

other specified diseases of the upper respiratory tract


pneumonia, organism unspecified


pneumonia, unspecified organism (*Required to code associated influenza first, if applicable [J09.X1, J10.0-, J11.0-].)


acute bronchitis


acute bronchitis, unspecified (There are also more specific ICD-10 codes, if applicable, e.g., acute bronchitis due to parainfluenza virus [J20.4], acute bronchitis due to respiratory syncytial virus [J20.5], acute bronchitis due to rhinovirus [J20.6], etc.)


pain in joint, lower leg


pain in right knee

pain in left knee

pain in unspecified knee


congestive heart failure, unspecified


heart failure, unspecified (*There are also more specific ICD-10 codes, if applicable, e.g., left ventricular failure [I50.1], acute systolic (congestive) heart failure [I50.21], chronic systolic (congestive) heart failure [I50.22], etc.)


unspecified hypothyroidism


hypothyroidism, unspecified

(*There are more specific ICD-10 codes, if applicable, e.g., congenital hypothyroidism with diffuse goiter [E03.0] or congenital hypothyroidism without goiter [03.1].)

Reviewing such data can be critical in collaborative decision-making with patients at the center of and as part of the discussion. This small example shows the value of taking advantage of ICD-10’s specificity when diagnosing patients and billing visit encounters via Practice Fusion enabling you to generate and evaluate such data to learn more about your patients’ needs, to ensure optimal quality of care, and to fulfill quality improvement measures critical to your patients.


  1. Kurusz S, Rubin C, Morisy LR. Bulletin of the American College of Surgeons. The transition to ICD-10 before October 1 compliance deadline. June 1, 2015. Accessed September 3, 2021.
  2. Blue Cross Blue Shield of Michigan. Primary Care ICD-10-CM Coding Tip Sheet: Overview of key chapter updates for primary care (primary care here includes internal medicine, family practice, and general practice). Accessed September 3, 2021. []
  3. American Medical Association. Fact Sheet: Preparing for the ICD-10 code set October 1, 2015 compliance date. The differences between ICD-9 and ICD-10. Last updated October 2, 2014. Accessed September 3, 2021.
  4. Definitive Healthcare, LLC. 10 most common diagnoses and procedures in primary care. Last accessed on September 3, 2021.
  5. American Academy of Professional Coders (AAPC). Fast forward: ICD:10 top 50 codes, family practice. Last accessed on September 3, 2021.