Orthopedics

The Best Diagnostic Tools for Orthopedic Infection Patients

MicroGenDX offers the ideal synergy of evidence-based diagnostics for periprosthetic joint infection

MicroGenDX OrthoKEY PJI OR and OrthoKEY PJI Clinic offer you the essential diagnostic compliment to culture that combines biomarkers for infection with molecular analysis – at the lowest cost and fastest turnaround on the market.

In contrast to what actually grows in culture, next-generation sequencing (NGS) identifies both the growth potential of clinically relevant microbes in a sample, and the relative abundance of those microbes. NGS analysis was recommended by a 2018 International Consensus Meeting strong consensus for identifying culture-negative infections — which NGS identifies 80-90% of the time.

MicroGenDX’s NGS protocol includes both positive and negative controls. Positive controls consist of known species and predictable results that can be verified in each stage of the process — ensuring each stage is performed correctly. The negative control consists of sterile water, run as a parallel sample to confirm that no contamination has occurred during any stage of the process.

The Case for NGS

The emerging gold standard for diagnosing chronic joint infection

Second generation 16S/ITS DNA sequencing produces data in vastly larger amounts, at significantly lower cost, in shorter time, and with less manual intervention than previous sequencing methods.

Unlike polymerase chain reaction (PCR), NGS is hypothesis-free and does not rely on a predetermined panel of PCR primer targets. NGS is capable of characterizing all microbial DNA present within a clinical sample and providing a complete picture of the microbial profile, without the need for preconceived ideas of the possible responsible pathogens.

MicroGenDX’s NGS searches against a curated microbial database of over 50,000 species for a match — including bacteria, yeast, and fungi — without the need for additional individual testing.

“We are standing at a very critical historic moment when molecular technologies can help us detect infecting organisms. NGS picks up organisms when cultures are negative and also provide us with the information much faster so during a critical period you are treating the patient with the right antibiotics. Mycobacterm abscesses is not an organism that we usually treat. Not having that information in your hands for 21 days…that could compromise the care of that patient. That is critical information.”

– Dr. Javad Parvizi, Orthopedic Surgeon, Director of Clinical Research at the Rothman Institute

NGS identifies Kocuria missed by MALDI-TOF in joint infection

A 77-year-old male with history of type 2 diabetes mellitus, hypertension, and hyperlipidemia presented to clinic with pain and stiffness to his right knee. Surgical history included a right TKA and subsequent revision surgery. Biomarkers from synovial fluid and serum were consistent with infection.

Synovial fluid and intraoperative samples were sent for culture MALDI-TOF and MicroGenDX NGS identification. Culture MALDI-TOF isolated very light growth Staphylococcus spp. and gram positive cocci. However, MicroGenDX NGS identified Kocuria ocularis to be the main culprit.  A research MALDI-TOF database confirmed NGS identification of Kocuria ocularis.  Patient treatment changed from vancomycin to penicillin, and patient was recovering well at 6-month follow-up.

The reference database used for MALDI-TOF did not contain Kocuria ocularis. An inability to identify an organism in this manner, along with the morphologic similarity to some coagulase-negative Staphylococcus (CoNS) species, led to the inaccurate reporting of the sample and initial implementation of incorrect antibiotic treatment.

The Limitations of Culture

“Many times these patients are culture negative. That is a big problem in our practice and in general orthopedics, is that we know they are infected, we see they are infected, but we can’t pin down the cause of the infection.”

— Dr. Jon E. Minter, DO, Arthritis & Joint Specialists, Alpharetta, GA

Why do the limitations of culture matter for periprosthetic joint infection (PJI)?

Sensitivity of routine culture in identifying infecting organisms in PJI is low, ranging from 39% to 70%. Negative culture PJI is associated with poorer outcomes, and the rate of treatment failure for culture negative PJI can be as high as 69%. Contrary to a widely held assumption, polymicrobial instead of monomicrobial infections make up the majority of PJI, but culture is not proficient at identifying multiple organisms in each sample. Incomplete treatment resulting in chronic infection may account for 70% of revision failures. The mortality and cost associated with PJI treatment are high and a 5-year mortality rate of 25%.

Instead of relying only on either culture or NGS, why not use both to maximize the probability of treatment success? NGS has high concordance with traditional culture, 96% based on a 2018 study published in the Bone and Joint Journal, and higher sensitivity than culture — some 90% vs 60% based on another Bone and Joint Journal published study (see references below). MicroGenDX is the only molecular testing lab that has published concordance data with PJI culture results.

Diagnosing Biofilm Infections

Polymicrobial infections are associated with poorer outcomes, but are vastly underestimated by culture methods

Biofilms foster tolerance to both antibiotics and host immune defenses, and multiple factors complicate biofilm chronic joint infection diagnosis and therefore treatment, including:

  • Biofilm growth characteristics

  • Sampling tools and techniques

  • Inherent limitations of culture — such as difficulty growing biofilms, VNBC bacteria, and delayed incubation of specimens

Next-gen DNA sequencing (NGS) can identify the multiple bacteria and fungi that make up a biofilm, which is a first step in addressing this challenge

  1. Microorganisms in free-living planktonic state.

  2. In seconds – Planktonic cells initiation attachment to each other.

  3. Within 24 hours – Microbes build a protective community with an extracellular matrix (EPS) of polysaccharides, proteins, and glycoproteins. Horizontal gene transfer strengthens antimicrobial resistance for the community.

  4. Within 72 hours – Biofilm matures. Aerobic and anaerobic bacteria occupy spaces with differed O2 levels, and nutrient limitations of the biofilm’s interior induce slow-growth or no-growth states.

  5. Bacteria will continually detach individually – or in clumps that can retain antimicrobial resistance – and establish other biofilm colonies.