Testing for knee arthritis involves a combination of a physical exam, imaging, and sometimes blood work or fluid analysis. There isn’t a single test that confirms it. Instead, your doctor pieces together findings from several sources to determine whether you have arthritis, what type it is, and how far it’s progressed.
The Physical Exam
A knee arthritis evaluation starts with your doctor’s hands. They’ll bend and straighten your knee through its full range of motion, feeling for crepitus, the grinding or crackling sensation that happens when roughened cartilage surfaces move against each other. Crepitus on its own isn’t necessarily a problem. It becomes significant when it’s accompanied by tenderness or pain, which points toward cartilage damage.
Your doctor will also press along the joint line, the seam where the thighbone meets the shinbone, checking for tenderness and swelling. They’ll look for bony enlargement around the joint, which signals that your body has been building extra bone in response to cartilage loss. The American College of Rheumatology uses a specific set of clinical markers to identify knee osteoarthritis: crepitus combined with morning stiffness lasting 30 minutes or less, or bony enlargement of the joint, can be enough to make the diagnosis even before imaging.
If your doctor suspects a cartilage or meniscus tear rather than (or in addition to) arthritis, they’ll perform targeted maneuvers. One common test involves pressing down on your kneecap and sliding it back and forth to check for pain underneath. Others involve rotating and extending your leg while pressing on the sides of the joint to detect clicks or pops that suggest a torn meniscus.
What X-Rays Reveal
X-rays are the standard first-line imaging test for knee arthritis. They’re fast, inexpensive, and show the hallmark signs of osteoarthritis clearly. Your doctor is looking for four things:
- Joint space narrowing: the gap between your thighbone and shinbone shrinks as cartilage wears away
- Osteophytes: bony spurs that form along the edges of the joint
- Subchondral sclerosis: increased bone density just beneath where cartilage used to be, caused by bone rubbing against bone
- Subchondral cysts: fluid-filled cavities that develop in the bone as the disease progresses
These findings are graded on the Kellgren-Lawrence scale, a 0 to 4 system that tells you how advanced the arthritis is. Grade 0 means a normal knee. Grade 1 is borderline, with possible early spur formation. Grade 2 shows definite bone spurs and possible narrowing. Grade 3 means moderate arthritis with clear narrowing, multiple spurs, and some hardening of the bone. Grade 4 is severe: large spurs, significant loss of joint space, dense bone changes, and visible deformity. Knowing your grade helps guide treatment decisions, from exercise and physical therapy at lower grades to joint replacement conversations at grade 3 or 4.
When MRI or Ultrasound Is Needed
X-rays show bone well but can’t directly visualize cartilage, soft tissue, or the lining of the joint. If your symptoms don’t match what the X-ray shows, or if your doctor suspects early-stage arthritis that hasn’t yet caused visible bone changes, an MRI may be ordered. MRI can detect cartilage thinning, bone marrow lesions, and meniscal tears that X-rays miss entirely. It’s particularly useful when someone has significant knee pain but a normal-looking X-ray.
Ultrasound plays a different role. It’s especially helpful for detecting inflammation inside the joint. On ultrasound, a doctor can see synovial hypertrophy (thickening of the joint lining) and increased blood flow to inflamed tissue, both signs of inflammatory arthritis like rheumatoid arthritis. Ultrasound can also pick up fluid buildup in the joint and help distinguish inflammatory arthritis from osteoarthritis or mechanical pain. It’s sometimes used to guide a needle into the joint for fluid sampling.
Blood Tests for Inflammatory Arthritis
If your doctor suspects your knee pain comes from an inflammatory type of arthritis rather than the wear-and-tear variety, blood work becomes essential. Osteoarthritis doesn’t cause abnormal blood tests, so these are really about ruling in or ruling out other conditions.
Two inflammation markers are typically checked. ESR (sed rate) and CRP both measure general inflammation levels in the body. Elevated results suggest something systemic is going on. From there, more specific tests help narrow the diagnosis. Rheumatoid factor and anti-CCP antibodies are the key markers for rheumatoid arthritis. Anti-CCP is particularly useful because it’s more specific to RA than rheumatoid factor, which can be elevated in other conditions too.
If your blood tests come back normal but your knee is swollen, warm, and painful, that doesn’t rule out inflammatory arthritis entirely. Some people with early RA test negative for these markers initially. Your doctor may repeat testing later or move on to joint fluid analysis.
Joint Fluid Analysis
Sometimes the most direct way to figure out what’s happening inside a knee is to take a sample of the fluid. This procedure, called arthrocentesis, involves inserting a needle into the joint and withdrawing a small amount of synovial fluid. It sounds worse than it feels for most people; local numbing is used, and the whole thing takes a few minutes.
The lab examines the fluid for white blood cell count and checks for crystals under a microscope. The white cell count tells a clear story. Osteoarthritis produces non-inflammatory fluid with fewer than 2,000 white blood cells per cubic millimeter. Inflammatory arthritis (including rheumatoid arthritis, gout, and pseudogout) pushes that count above 2,000 and sometimes up to 50,000. Counts above 50,000 raise concern for a joint infection, which requires urgent treatment.
Crystal analysis is what definitively diagnoses gout and pseudogout. Urate crystals mean gout. Calcium pyrophosphate crystals mean pseudogout. This distinction matters because gout and pseudogout in the knee can mimic other forms of arthritis, and each requires a different treatment approach. Without fluid analysis, these conditions can be misdiagnosed for months.
Self-Assessment Questionnaires
Doctors often use standardized questionnaires to measure how arthritis affects your daily life. The most widely used is the WOMAC index, a 24-question survey covering three areas: pain (5 questions), stiffness (2 questions), and physical function (17 questions). Each question is scored from 0 to 4, with lower scores meaning fewer symptoms. You might be asked how much pain you feel climbing stairs, how stiff your knee is in the morning, or how difficult it is to get in and out of a car.
These scores don’t diagnose arthritis on their own, but they establish a baseline. If you’re tracked over time or considering surgery, your WOMAC score gives your care team a concrete way to measure whether you’re getting better or worse. It also helps capture something imaging can’t: how much the arthritis actually interferes with your life, since X-ray severity and symptom severity don’t always match up.
Putting the Diagnosis Together
No single test tells the full story. A knee with grade 3 changes on X-ray might belong to someone with minimal pain, while a knee that looks nearly normal on imaging might be the source of daily discomfort. That’s why diagnosis relies on layering the physical exam, imaging, and lab results together.
For most people, the process starts with an exam and X-rays. If those point clearly to osteoarthritis, no further testing is needed. If there’s swelling, warmth, or signs of inflammation, blood work and possibly fluid analysis follow. MRI is reserved for cases where the diagnosis is unclear or surgery is being considered. The type of arthritis identified, whether osteoarthritis, rheumatoid arthritis, gout, or pseudogout, determines everything about the treatment path ahead.