Plain Language Summary

Diagnosis of Periprosthetic Joint Infections (PJI) of the Hip and Knee


This plain language summary provides an overview of the diagnosis of periprosthetic joint infections (PJI) of the hip and knee.   

What is a periprosthetic joint infection (PJI)?

“Periprosthetic” simply means “around an implant” (such as an artificial knee or hip joint). “Joint infection” is bacteria or “bugs” within the joint. 

If an infection is not diagnosed, the chance that the artificial joint can fail is high.  Testing for infection should be based on whether patients have a high or low chance of having an infection, as determined by the following:

Higher Probability of Infection

One or more symptoms, AND at least one or more:

1) risk factor  OR

2) physical exam finding  OR

3) early implant loosening

Lower Probability of Infection

Pain or joint stiffness only and none of the following:

Risk factors, physical exam findings, or early implant loosening

The following factors were not found to be risk factors for PJI in patients who have had a total joint replacement:

  • smoking
  • obesity (in patients with artificial knees)
  • the use of a drain (a temporary tube coming out of the joint)
  • conditions that harm or weaken the immune system (defined below) in patients with a new hip.  

However, several days of wound drainage and hematoma (excessive blood around the wound after surgery) might increase the chance of having a PJI.

Conditions that harm or weaken the immune system:

  • HIV, diabetes, hepatitis, chemotherapy or other similar medication, autoimmune diseases, inflammatory arthritis, renal disease, liver failure, malnourishment, sickle cell disease, hemophilia, and solid organ transplant

Factors to help determine risk of infection

  • prior infection of the joint
  • superficial surgical site infection (hip and knee)
  • longer OR time (>2.5 hours)
  • immunocompromising states in patients with a knee replacement (see above)
  • any recent (<1yr) infection of the blood stream
  • skin conditions
  • IV drug use
  • recent (<3 year) MRSA infection
  • active infection in another location
  • early (<5 yrs.) implant loosening.
  • Pain or stiffness in the replaced joint
  • Warmth, redness, swelling or sinus tract (a wound that opens up and ‘tunnels’ to the skin) of the joint

Testing for PJI

There is strong evidence to support two blood tests for patients assessed for PJI: erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP).  However, there is not good evidence on whether special imaging tests (such as CT or MRI) help diagnose PJI.

There is also strong evidence against the use of intraoperative Gram stain (a special lab test) to rule out PJI.

Knee Aspiration

There is strong evidence to support joint aspiration (taking fluid from the joint) in patients with abnormal blood test (ESR and/or CRP) results.  The fluid should be sent for testing: a culture (growing the fluid in a flask), a white blood cell count of the fluid, and a differential (a special percentage of white cells).

Either synovial fluid white blood cell count over 1700cells/μl OR neutrophil (a special type of white cell) percentage greater than 65% is highly suggestive of PJI.

Hip Aspiration

A selective approach should be taken to aspiration, or taking fluid from, the hip based on the patient’s likelihood of having a PJI, and the results of the ESR AND CRP. Repeat hip aspiration should only occur when there is a discrepancy between the probability of PJI and the initial aspiration culture result. For example, a high probability of developing a PJI and a negative culture result or a low probability and a positive culture result. 

Antibiotics and Cultures

Patients should be off antibiotics for a minimum of 2 weeks prior to obtaining a joint culture.

Intra-Operative Procedures

There is strong evidence to support the use of frozen sections of joint tissues (this is a special test where a doctor looks under the microscope at the tissue) in patients who are undergoing surgery again for whom the diagnosis of PJI has either not been proven or ruled out.

Insufficient information is available to determine the efficacy of frozen sections in patients with an underlying inflammatory arthropathy (a special type of inflammation of the joints).

There is also strong evidence that multiple cultures be obtained at the time of repeat surgery in patients being assessed for PJI.


There is no single test that can reliably diagnose an infection. Diagnosis usually depends on the combined use of many tests.