| A&O | Alert and oriented |
| A&P | Auscultation and percussion; anterior and posterior; assessment and plan |
| A&W | Alive and well |
| A1C | Hemoglobin A1C |
| aa | Of each (ana) |
| AA | Aplastic anemia; Alcoholics Anonymous |
| AAA | Abdominal aortic aneurysm |
| AAL | Anterior axillary line |
| AAO | Awake, alert, and oriented |
| AAP | American Academy of Pediatrics |
| ABC | Absolute band count; absolute basophil count; aspiration, biopsy, and cytology; artificial β-cells |
| Abd | Abdomen |
| ABG | Arterial blood gases |
| ABI | Ankle brachial index |
| ABP | Arterial blood pressure |
| ABW | Actual body weight |
| ABx | Antibiotics |
| AC | Before meals (ante cibos) |
| ACC | American College of Cardiology |
| ACE | Angiotensin-converting enzyme |
| ACEI | Angiotensin-converting enzyme inhibitor |
| ACG | American College of Gastroenterology |
| ACL | Anterior cruciate ligament |
| ACLS | Advanced cardiac life support |
| ACR | Albumin:creatinine ratio; American College of Rheumatology |
| ACS | Acute coronary syndrome |
| ACT | Activated clotting time |
| ACTH | Adrenocorticotropic hormone |
| AD | Alzheimer disease, right ear (auris dextra) |
| ADA | American Diabetes Association; adenosine deaminase |
| ADE | Adverse drug effect (or event) |
| ADH | Antidiuretic hormone |
| ADHD | Attention-deficit hyperactivity disorder |
| ADL | Activities of daily living |
| ADR | Adverse drug reaction |
| AED | Antiepileptic drug(s) |
| AF | Atrial fibrillation |
| AFB | Acid-fast bacillus; aortofemoral bypass; aspirated foreign body |
| Afeb | Afebrile |
| AFP | α-Fetoprotein |
| A/G | Albumin–globulin ratio |
| AGA | American Gastroenterological Association |
| AGE | Acute viral gastroenteritis |
| AHA | American Heart Association |
| AI | Aortic insufficiency |
| AIDS | Acquired immunodeficiency syndrome |
| AKA | Above-knee amputation; alcoholic ketoacidosis; all known allergies; also known as |
| AKI | Acute kidney injury |
| ALD | Alcoholic liver disease |
| ALFT | Abnormal liver function test |
| ALK | Anaplastic lymphoma kinase |
| ALL | Acute lymphocytic leukemia; acute lymphoblastic leukemia |
| ALP | Alkaline phosphatase |
| ALS | Amyotrophic lateral sclerosis |
| ALT | Alanine aminotransferase |
| AMA | Against medical advice; American Medical Association; antimitochondrial antibody |
| AMI | Acute myocardial infarction |
| AML | Acute myelogenous leukemia |
| Amp | Ampule |
| AMPA | α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid |
| ANA | Antinuclear antibody |
| ANC | Absolute neutrophil count |
| ANLL | Acute nonlymphocytic leukemia |
| Anti-CCP | Anticyclic citrullinated peptide |
| AODM | Adult-onset diabetes mellitus |
| A&O × 3 | Awake and oriented to person, place, and time |
| A&O × 4 | Awake and oriented to person, place, time, and situation |
| AOM | Acute otitis media |
| AP | Anteroposterior |
| APACHE | Acute Physiology and Chronic Health Evaluation |
| APAP | Acetaminophen (N-acetyl-p-aminophenol) |
| aPTT | Activated partial thromboplastin time |
| ARB | Angiotensin receptor blocker |
| ARC | AIDS-related complex |
| ARDS | Adult respiratory distress syndrome |
| ARF | Acute renal failure; acute respiratory failure; acute rheumatic fever |
| AROM | Active range of motion |
| AS | Left ear (auris sinistra) |
| ASA | Aspirin (acetylsalicylic acid) |
| ASCVD | Atherosclerotic cardiovascular disease |
| ASD | Atrial septal defect |
| ASH | Asymmetric septal hypertrophy |
| ASHD | Arteriosclerotic heart disease |
| AST | Aspartate aminotransferase |
| ATG | Antithymocyte globulin |
| ATN | Acute tubular necrosis |
| AU | Each ear (auris uterque) |
| AV | ...