What Tests Are Done Before Knee Replacement Surgery?

Before knee replacement surgery, you can expect a combination of imaging studies, blood tests, infection screening, and functional assessments designed to confirm that the procedure is appropriate and that your body is ready for it. Some of these tests establish whether surgery is the right call in the first place, while others flag hidden risks that could derail your recovery. The full battery varies by surgeon and hospital, but certain evaluations have become standard practice because the evidence consistently links them to better outcomes.

Imaging Studies That Confirm the Need for Surgery

The process almost always begins with X-rays. Standing (weight-bearing) knee X-rays are the cornerstone of preoperative imaging because they show how the joint looks under the load it actually carries day to day. A non-weight-bearing X-ray, taken while you’re lying down, can make the joint space look deceptively healthy. Research has shown that medial joint space width on weight-bearing films can be as narrow as half a millimeter in advanced osteoarthritis, compared to slightly wider readings on non-weight-bearing images, and that a narrower joint space along with certain alignment angles is strongly associated with rapid disease progression.¹ Your surgeon uses these images to grade the arthritis, measure leg alignment, and decide whether you’ve reached the point where conservative treatments are no longer enough.

Most people only need standard X-rays, but some surgeons now order a preoperative CT scan as well. CT-based three-dimensional imaging lets the surgical team predict component sizes more accurately and plan the positioning of the implant before they ever enter the operating room.² This is especially common when robotic-assisted surgery is planned, because the robot relies on a 3D model of your knee built from CT data to guide bone cuts and implant placement.³ If your surgeon uses conventional instruments without robotic assistance, a CT scan is less likely to be part of the workup, though practices vary. An MRI is occasionally ordered too, usually to evaluate soft tissue damage or to investigate symptoms that don’t fully match the X-ray findings, but it isn’t routine before a straightforward knee replacement.

Routine Blood Work and What Surgeons Are Looking For

You’ll have blood drawn at a preoperative visit, typically a few weeks before surgery. The basic panel usually includes a complete blood count to check for anemia or signs of infection, a metabolic panel to evaluate kidney and liver function, and a coagulation screen to make sure your blood clots normally. These aren’t exotic tests. They’re the same ones ordered before most major surgeries, and they catch problems that could cause dangerous bleeding, delayed wound healing, or complications from anesthesia.

What makes the pre-knee-replacement blood panel distinct is the added attention to a few specific markers. If you have diabetes or if your fasting blood sugar is elevated, your surgeon will almost certainly check your hemoglobin A1c (HbA1c), a measure of average blood sugar over the previous two to three months. A meta-analysis found that high HbA1c and elevated perioperative blood glucose are both associated with a higher risk of deep infection around the new joint.⁴ More recent work has questioned whether HbA1c alone is a precise predictor, noting its utility is somewhat limited as a standalone number, but it remains a widely used and accessible biomarker for preoperative risk stratification.⁵ In practical terms, if your HbA1c is significantly elevated, your surgeon may postpone the procedure until your blood sugar is better controlled.

Nutritional Screening and Why It Matters

This one surprises many patients. Your nutritional status can meaningfully affect how well you heal, and surgeons increasingly test for it before joint replacement. Serum albumin is the most commonly used marker because low albumin correlates with protein-energy malnutrition, which worsens the body’s ability to handle the stress of surgery. Some centers also check prealbumin, transferrin, and total lymphocyte count to get a fuller picture. Malnourished patients heading into knee or hip replacement have been shown to face longer hospital stays, higher reoperation rates, more infections, and even higher mortality.⁶

If your albumin comes back low, the solution isn’t necessarily to cancel surgery. Many programs use a brief “prehabilitation” period where you focus on protein-rich nutrition and sometimes oral supplements to bring your levels up before the operation. The test itself is inexpensive and easy to add to the standard blood draw, which is why it has become more common even in patients who don’t look malnourished on the surface. Obesity, for example, doesn’t rule out nutritional deficiency, because it’s possible to carry excess calories while still lacking adequate protein or micronutrients.

Screening for Hidden Infections

Joint replacement introduces a large metal-and-plastic implant into your body, and bacteria that would cause a trivial problem in everyday life can colonize that implant and create a devastating infection called periprosthetic joint infection. Because of this, surgeons go out of their way to find and treat infections you might not even know you have.

Nasal Swabs for Staphylococcus

About one in four people carry Staphylococcus aureus in their nose without any symptoms. Some carry the drug-resistant strain, MRSA. A nasal swab, which takes only seconds, can identify carriers through a rapid test. Patients who test positive are treated before surgery with mupirocin nasal ointment and chlorhexidine body washes, a decolonization protocol that has shown encouraging results in reducing surgical site infections after knee replacement.⁷ Rapid detection of MRSA in particular allows surgeons to identify high-risk patients and tailor their antibiotic prophylaxis accordingly.⁸ This is now a standard part of the preoperative protocol at most major joint replacement centers.

Urine Tests

Urinalysis before knee replacement is common, but the evidence supporting routine screening in people with no urinary symptoms is surprisingly thin. Despite this, the practice has become embedded in clinical workflows in many countries on the theory that even a silent urinary tract infection could seed bacteria into the bloodstream during surgery.⁹ In practice, if your urinalysis is clean, nothing changes. If it comes back positive and you have symptoms, you’ll be treated with antibiotics before surgery proceeds. The debate is really about patients with positive urine cultures but no symptoms, a grey area where different surgeons handle things differently.

Dental Clearance

Many orthopedic practices ask you to see your dentist before knee replacement. The concern is that dental infections or procedures like scaling can cause transient episodes of bacteria entering the bloodstream, and those bacteria could theoretically settle on the new implant.¹⁰ In practice, you’ll be asked to resolve any active dental infections, cavities, or abscesses before the operation. Some surgeons require a signed clearance letter from your dentist; others simply ask whether you have any known dental issues. The strength of evidence behind mandatory dental clearance is debated, but it remains a common requirement.

Cardiac and Pulmonary Evaluation

Your heart and lungs need to be in good enough shape to tolerate both the anesthesia and the physical stress of surgery and recovery. Almost everyone gets an electrocardiogram (ECG) as part of the preoperative workup. If you have a history of heart disease, a recent change in symptoms, or significant risk factors like long-standing diabetes or prior heart attacks, your surgeon may refer you to a cardiologist for a more detailed evaluation. This could include an echocardiogram or a stress test, depending on what the cardiologist thinks is warranted.

Pulmonary evaluation is less routine but comes into play if you have a known lung condition like COPD, are a heavy smoker, or have severe obesity. A chest X-ray is sometimes ordered as a baseline. If you use a CPAP machine for sleep apnea, make sure your surgical team knows, because undiagnosed or poorly managed sleep apnea can complicate anesthesia and recovery. Your anesthesiologist will review all of this before the day of surgery, and in some cases may ask for pulmonary function tests to gauge how well your lungs are working.

Metal Allergy Testing

Standard knee implants contain metals like cobalt, chromium, and nickel. If you have a known history of metal allergy, such as reactions to costume jewelry, belt buckles, or watch backs, your surgeon may order skin patch testing before choosing the implant. Patch testing remains the most common method for diagnosing metal hypersensitivity, and evidence supports its use in patients with a reported history of metal allergy to guide prosthesis selection.¹¹ Patients who test positive for nickel or other metals can receive hypoallergenic implants coated with materials like titanium niobium or oxidized zirconium that avoid the offending allergen.

Routine patch testing of all knee replacement candidates is not standard practice, because true implant-related metal hypersensitivity is uncommon. If you have no history of reactions to metals, testing isn’t typically warranted. But if you do have a history and your surgeon doesn’t bring it up, mention it yourself. The test is straightforward: small patches containing various metal allergens are applied to your back, left for about 48 hours, and then read for a reaction.

Blood Clot Risk Assessment

Knee replacement carries a well-known risk of venous thromboembolism (blood clots in the deep veins of the leg or, more dangerously, the lungs). Virtually every patient receives some form of blood-thinning medication after surgery to reduce this risk. However, the ideal type and dose of blood thinner depend on your individual risk profile. Researchers have developed scoring systems that weigh factors like your age, body mass index, history of prior clots, and other medical conditions to sort patients into lower-risk and higher-risk categories.¹²

For you as a patient, this assessment usually happens during the preoperative visit or the surgical planning conference, not as a separate appointment. Your surgeon reviews your medical history and any relevant blood work (like clotting studies) and decides whether standard-dose prophylaxis is enough or whether more aggressive anticoagulation is needed. If you have a personal or family history of blood clots, or if you take estrogen-containing medications, bring it up early, because it may affect both the timing and type of blood thinner you receive.

Functional Baseline Measurements

Before fixing a knee, the surgical team wants to know exactly how bad things are right now. This serves two purposes: it confirms that you’ve reached a threshold of disability that justifies the risks of surgery, and it establishes a baseline to measure improvement afterward. The most common approach is a patient-reported outcome measure, essentially a questionnaire you fill out about your pain, stiffness, and ability to do everyday tasks.

The Oxford Knee Score is one of the most widely used, with research confirming it is well-suited for evaluating knee function both before and after replacement.¹³ Other commonly used tools include the Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) and the Knee Society Score.¹⁴ These aren’t “tests” in the blood-draw sense, but they’re a formal part of the preoperative evaluation and may influence whether your surgeon recommends proceeding. If your questionnaire scores suggest relatively mild disability, your surgeon might counsel you to try other treatments first and revisit the idea of surgery later.

Psychological and Mental Health Screening

This is an area that has gained significant traction in the last decade. Research consistently shows that your psychological state going into knee replacement predicts how satisfied you’ll be with the outcome. One study found that over half of patients had reduced mental wellbeing before surgery, and those with poor preoperative mental health scores had significantly worse functional outcomes two years later compared to those who started in a better psychological place.¹⁵

A growing body of evidence implicates anxiety, depression, fear of movement, and catastrophic thinking about pain as negative prognostic factors following knee replacement.¹⁶ Not every surgical practice formally screens for these factors yet, but many now include mental health questionnaires in their preoperative packet. The point isn’t to deny you surgery if you’re anxious or depressed. It’s to identify patients who might benefit from cognitive behavioral therapy, counseling, or optimized medication management before or alongside surgery to give them the best chance of a good result.

If you find yourself dreading surgery to the point of significant distress, or if chronic pain has left you feeling hopeless, bringing this up with your surgical team can actually help. Addressing these issues before the operation doesn’t delay surgery by much and can make a real difference in how you experience your recovery.

What Happens at the Preoperative Medical Clearance Visit

Several weeks before surgery, you’ll typically see either your primary care doctor or a perioperative medicine specialist for what’s often called “medical clearance.” This visit ties together many of the individual tests described above. The doctor reviews your full medical history, medication list, and test results and writes a letter to the surgeon confirming that you’re safe to proceed or identifying conditions that need to be optimized first.

At this visit you can expect to discuss:

Medications to stop: blood thinners, certain anti-inflammatory drugs, and some supplements like fish oil need to be paused before surgery. Your clearance doctor and surgeon will coordinate on timing.
Medications to continue: most blood pressure, thyroid, and heart medications continue right up to and including the morning of surgery, often taken with a small sip of water.
Anesthesia considerations: the anesthesiologist may reach out separately if your airway, lung function, or cardiac history raises any flags.
Prior surgical complications: if you’ve ever had a bad reaction to anesthesia, a wound infection, or a blood clot after a previous surgery, this is when it gets documented formally.

This visit also serves as a safety net. If any test result slipped through the cracks or if a new symptom has appeared since the surgeon last saw you, the clearance doctor catches it. Occasionally a previously undiagnosed condition turns up during this evaluation, such as atrial fibrillation picked up on the ECG or unrecognized kidney dysfunction on the metabolic panel. These findings don’t always cancel surgery, but they do shape how the anesthesia team manages you in the operating room and afterward.

Tests You Might Expect but Don’t Always Need

Patients often assume they’ll need an MRI before knee replacement. In most cases, the diagnosis is established with plain X-rays and a physical exam, and an MRI adds little value when the arthritis is clearly advanced. MRI is more useful when the diagnosis is uncertain or when the surgeon suspects a different problem masquerading as straightforward arthritis.

Similarly, stress tests and echocardiograms are not required for everyone. If you’re in generally good health, walk regularly without chest pain or shortness of breath, and have no cardiac history, a resting ECG alone is usually sufficient. Ordering cardiac tests on every patient drives up cost without improving safety in low-risk individuals. The same goes for pulmonary function tests, which are reserved for patients with known or suspected lung disease, not applied universally.

Routine preoperative urinary screening is another area where practice has outpaced evidence. While many hospitals include it in their standard preoperative order set, systematic reviews have questioned whether it benefits patients who have no urinary symptoms. If your hospital orders it, there’s no harm in the test itself, but a positive result in someone without symptoms doesn’t necessarily mean surgery should be delayed.

How Far in Advance Testing Usually Happens

Most preoperative testing takes place two to four weeks before the surgery date. Blood work and urinalysis need to be recent enough to reflect your current health, but not so close to surgery that there isn’t time to address problems that surface. Imaging is often done earlier, sometimes months before, as part of the process that led to the decision to operate in the first place.

Nasal swab screening typically happens at the preoperative visit so that decolonization treatment with mupirocin and chlorhexidine washes can be completed in the days leading up to surgery. Dental clearance, if required, should be taken care of well in advance, since resolving an active dental problem can take weeks. And if your HbA1c comes back elevated, you may need several weeks to months of adjusted diabetes management before your surgeon is comfortable proceeding.

The overall timeline can feel rushed or drawn out depending on your health. A relatively healthy person with well-controlled conditions might breeze through all the testing in one or two appointments. Someone with multiple chronic conditions might go through several rounds of specialist consultations and repeat lab work before everything lines up. Either way, the testing isn’t busywork. Each piece gives the surgical team information that directly shapes how they manage your anesthesia, your antibiotic regimen, your blood-clot prevention, and your recovery plan.