PD is a medical abbreviation with several different meanings depending on the context. The most common uses are Parkinson’s disease (in neurology), peritoneal dialysis (in kidney care), progressive disease (in cancer treatment), and pupillary distance (in eye care). Because the same two letters show up across so many specialties, the meaning always depends on where and how it’s being used.
Parkinson’s Disease
Parkinson’s disease is one of the most recognized meanings of PD in medicine. It’s a progressive neurological condition that primarily affects movement. Diagnosis requires the presence of bradykinesia, which is a noticeable slowness in initiating and carrying out movements, plus at least one additional motor symptom: either a resting tremor or muscle rigidity. These are the core criteria established by the Movement Disorder Society.
Beyond the hallmark movement problems, Parkinson’s disease also involves non-motor symptoms that can appear years before tremor or stiffness. Loss of smell is one early sign and is actually used as a supportive criterion in diagnosis. Other non-motor features include sleep disturbances, constipation, mood changes, and cognitive difficulties that may develop as the disease progresses. When you see “PD” in a neurology report or research paper, it almost always refers to Parkinson’s disease.
Peritoneal Dialysis
In kidney care, PD stands for peritoneal dialysis, a treatment for people whose kidneys can no longer filter waste from the blood effectively. Unlike hemodialysis, which uses an external machine, peritoneal dialysis uses the lining of your own abdomen (called the peritoneum) as a natural filter. A cleansing fluid is introduced into the abdominal cavity through a small catheter. Toxins from the blood pass through the peritoneal membrane into that fluid, which is then drained and discarded after a set dwell time.
Peritoneal dialysis is often done at home, either manually several times a day or with a machine that runs cycles overnight while you sleep. This makes it appealing for people who want more flexibility in their daily routine compared to the in-center schedule of hemodialysis. If you encounter PD in the context of nephrology or kidney failure, this is the meaning.
Progressive Disease in Cancer Treatment
In oncology, PD means progressive disease, a specific classification used when evaluating how a tumor responds to treatment. Under the widely used RECIST guidelines (Response Evaluation Criteria in Solid Tumors), progressive disease is defined as at least a 20% increase in the combined diameter of target tumors compared to the smallest measurement recorded during treatment. That increase must also be at least 5 millimeters in absolute terms. The appearance of any new tumor also counts as progression, regardless of size changes in existing ones.
This designation matters because it directly influences treatment decisions. When imaging shows PD, oncologists typically consider switching to a different therapy, since the current approach is no longer controlling tumor growth. You’ll see this abbreviation on radiology reports and in clinical trial results alongside other response categories like CR (complete response), PR (partial response), and SD (stable disease).
Pupillary Distance
In optometry and ophthalmology, PD refers to pupillary distance: the measurement in millimeters between the centers of your two pupils. This number is essential for fitting prescription eyeglasses correctly. It ensures that the optical center of each lens lines up precisely with your pupil, so light bends through the lens the way it should. A misaligned lens can cause eye strain, headaches, or blurry vision.
You can measure your own PD using a millimeter ruler and a mirror, which is useful if you’re ordering glasses online. Average adult PD ranges from about 54 to 74 mm. Some prescriptions list two numbers (one for each eye measured from the bridge of the nose), called monocular PD, which provides a more precise fit.
Panic Disorder
In psychiatry, PD can refer to panic disorder, a condition defined by recurrent, unexpected panic attacks. A panic attack is an abrupt surge of intense fear that peaks within minutes and involves four or more physical or psychological symptoms, such as a racing heart, shortness of breath, chest pain, dizziness, or a feeling of losing control.
A single panic attack doesn’t qualify as panic disorder. The diagnosis requires that at least one attack is followed by a month or more of persistent worry about having another attack, or a significant change in behavior to avoid triggering one. That avoidance pattern, like skipping exercise or staying away from unfamiliar places, is often what brings people to seek help.
Pharmacodynamics
In pharmacology, PD stands for pharmacodynamics, the study of what a drug does to the body. This includes how a medication produces its effects at the molecular and cellular level, how strong those effects are at a given dose, and what side effects it causes. Pharmacodynamics is often paired with pharmacokinetics (PK), which covers the reverse: what the body does to the drug, including how it’s absorbed, distributed, broken down, and eliminated. Together, PK/PD studies help determine proper dosing. You’ll encounter this meaning primarily in research literature and drug development contexts.
Less Common Uses
PD occasionally appears in other medical contexts as well. Polydipsia, meaning excessive and persistent thirst, is sometimes abbreviated as PD. It can be a sign of uncontrolled diabetes, diabetes insipidus, or other hormonal conditions. In medical shorthand on prescriptions, “p.d.” has historically been used as an abbreviation for “per day,” though this usage has fallen out of favor because of the risk of confusion.
Because PD has so many potential meanings, medical professionals generally rely on context to interpret it. A note from a neurologist almost certainly means Parkinson’s disease. A lab report from a nephrology clinic means peritoneal dialysis. If you encounter PD on a medical document and aren’t sure which meaning applies, the specialty of the provider or the surrounding text will usually make it clear.