| A09 | Infectious gastroenteritis and colitis, unspecified | Infectious Disease | Stomach flu/viral gastroenteritis; specify organism when known. |
| B00.9 | Herpesviral infection, unspecified | Infectious Disease | HSV; use B00.1 for herpes simplex dermatitis, B00.2 for gingivostomatitis. |
| B34.9 | Viral infection, unspecified | Infectious Disease | General viral illness; use more specific codes when organism identified. |
| E03.9 | Hypothyroidism, unspecified | Endocrine | Common endocrine diagnosis; use E06.3 for autoimmune thyroiditis (Hashimoto's). |
| E11.65 | Type 2 diabetes mellitus with hyperglycemia | Endocrine | Indicates diabetes with documented hyperglycemia; important for demonstrating medical necessity of treatment adjustments. |
| E11.9 | Type 2 diabetes mellitus without complications | Endocrine | Use additional codes for specific complications: E11.65 (hyperglycemia), E11.21 (diabetic nephropathy), E11.40 (neuropathy). |
| E66.01 | Morbid (severe) obesity due to excess calories | Endocrine | BMI 40+; code BMI separately with Z68.4x codes. Often required for bariatric surgery authorization. |
| E78.5 | Hyperlipidemia, unspecified | Endocrine | General hyperlipidemia code; use E78.0 for pure hypercholesterolemia, E78.1 for hypertriglyceridemia. |
| F10.20 | Alcohol dependence, uncomplicated | Mental Health | Use additional codes for associated conditions; F10.21 for alcohol dependence in remission. |
| F32.1 | Major depressive disorder, single episode, moderate | Mental Health | Specify episode type and severity. F32.0 (mild), F32.2 (severe without psychotic), F33.x for recurrent episodes. |
| F41.1 | Generalized anxiety disorder | Mental Health | Common mental health diagnosis; distinguish from F41.0 (panic disorder), F41.9 (anxiety disorder, unspecified). |
| F90.0 | Attention-deficit hyperactivity disorder, predominantly inattentive type | Mental Health | ADHD-inattentive; F90.1 for predominantly hyperactive-impulsive, F90.2 for combined type. |
| G43.909 | Migraine, unspecified, not intractable, without status migrainosus | General | Use specific migraine codes when type is documented: G43.0 (without aura), G43.1 (with aura). |
| I10 | Essential (primary) hypertension | Cardiovascular | Most commonly billed diagnosis code; use for unspecified or primary hypertension. Do not use for hypertensive crisis (I16.x) or secondary hypertension. |
| I25.10 | Atherosclerotic heart disease of native coronary artery without angina pectoris | Cardiovascular | Coronary artery disease (CAD); use specific codes for bypass graft arteries or with angina. |
| I48.91 | Unspecified atrial fibrillation | Cardiovascular | Use more specific codes when type is known: I48.0 (paroxysmal), I48.1 (persistent), I48.2 (chronic/permanent). |
| I50.9 | Heart failure, unspecified | Cardiovascular | Use specific codes when type is known: I50.1 (left ventricular), I50.20-I50.23 (systolic), I50.30-I50.33 (diastolic). |
| J02.9 | Acute pharyngitis, unspecified | Respiratory | Sore throat; use J02.0 for streptococcal pharyngitis when confirmed by testing. |
| J06.9 | Acute upper respiratory infection, unspecified | Respiratory | Common cold/URI; one of the most frequently used diagnosis codes in primary care and urgent care settings. |
| J18.9 | Pneumonia, unspecified organism | Respiratory | Use when organism is unknown; code specific organism when identified (J13 for pneumococcal, J15.x for bacterial). |
| J44.1 | Chronic obstructive pulmonary disease with acute exacerbation | Respiratory | COPD exacerbation; J44.0 for COPD with acute lower respiratory infection, J44.9 for unspecified COPD. |
| J45.20 | Mild intermittent asthma, uncomplicated | Respiratory | Classify asthma severity: mild intermittent (J45.2x), mild persistent (J45.3x), moderate persistent (J45.4x), severe persistent (J45.5x). |
| K21.0 | Gastro-esophageal reflux disease with esophagitis | Gastrointestinal | GERD with esophagitis; K21.9 for GERD without esophagitis. |
| K58.9 | Irritable bowel syndrome without diarrhea | Gastrointestinal | IBS; K58.0 for IBS with diarrhea, K58.1 for IBS with constipation. |
| K80.20 | Calculus of gallbladder without cholecystitis | Gastrointestinal | Gallstones without inflammation; K80.00 with acute cholecystitis, K80.10 with chronic cholecystitis. |
| L50.9 | Urticaria, unspecified | General | Hives/allergic rash; specify L50.0 for allergic urticaria when allergy is documented. |
| M17.11 | Primary osteoarthritis, right knee | Musculoskeletal | Common in orthopedic and primary care; laterality required. M17.12 for left knee, M17.0 for bilateral. |
| M23.611 | Other spontaneous disruption of anterior cruciate ligament, right knee | Musculoskeletal | ACL tear; specify laterality and type of tear (partial vs. complete) when documented. |
| M25.511 | Pain in right shoulder | Musculoskeletal | Laterality must be specified for shoulder pain. Use M25.512 for left shoulder, M25.519 for unspecified. |
| M54.2 | Cervicalgia | Musculoskeletal | Neck pain; use M54.2 for pain without radiculopathy. Add M50.x codes for cervical disc disorders. |
| M54.5 | Low back pain | Musculoskeletal | One of the most common reasons for office visits; specify chronicity with M54.50 (unspecified) or laterality when known. |
| M79.3 | Panniculitis, unspecified | Musculoskeletal | Inflammation of subcutaneous fat; specify site when possible. |
| N39.0 | Urinary tract infection, site not specified | General | UTI unspecified; use B96.2x for specific organism. N30.0 for acute cystitis, N10 for acute pyelonephritis. |
| R05.9 | Cough, unspecified | Respiratory | Symptom code; link to underlying diagnosis when established. R05.1 for acute cough, R05.3 for chronic cough. |
| R10.9 | Unspecified abdominal pain | General | Use more specific codes when location is known: R10.0 (acute abdomen), R10.1 (upper), R10.3 (lower). |
| R51.9 | Headache, unspecified | General | Use as initial presentation; transition to specific diagnosis (migraine, tension) when confirmed. |
| Z00.00 | Encounter for general adult medical examination without abnormal findings | General | Annual physical exam; add Z-codes for screening services performed during the visit. |
| Z12.31 | Encounter for screening mammogram for malignant neoplasm of breast | General | Screening mammography; covered at 100% as preventive service under most plans. |
| Z23 | Encounter for immunization | General | Primary diagnosis for vaccine administration; add codes for specific vaccines given. |
| Z87.891 | Personal history of nicotine dependence | General | Former smoker/tobacco user; Z72.0 for current tobacco use, F17.2x for current nicotine dependence. |