The Karius blood test is a diagnostic tool designed to identify infections by detecting microbial DNA in a patient’s blood sample. It offers a non-invasive way to diagnose infections, providing insights into the presence of bacteria, fungi, parasites, and DNA viruses.
How the Karius Test Works
The Karius test operates by analyzing “cell-free DNA” (cfDNA) found in the bloodstream. These tiny fragments of DNA circulate freely after cells, including human cells and microbial cells, break down. The specific focus of the Karius test is “microbial cell-free DNA” (mcfDNA), which originates from pathogens causing an infection.
The process begins with a standard blood draw, typically a 5-mL sample collected in a plasma preparation tube. This blood sample is then sent to a specialized laboratory where the mcfDNA is isolated from the plasma. Following isolation, the test utilizes next-generation sequencing (NGS) to analyze these minute DNA fragments. NGS allows for the simultaneous sequencing of millions of DNA fragments, which are then compared against a vast database of microbial genomes.
This comparison identifies specific microbial signatures, revealing the presence and even the quantity of various pathogens. The Karius test is sometimes referred to as a “liquid biopsy” for infection, a term more commonly used in cancer diagnostics, because it can detect DNA from infection sites throughout the body from a simple blood sample. This includes infections localized in areas like the cerebrospinal fluid (CSF), lungs, or deep abscesses, where pathogen cfDNA can still be found in the plasma. The laboratory typically reports pathogen detection results within one day after sample receipt, with bacterial antimicrobial resistance (AMR) marker detection available the following day.
Conditions the Karius Test Identifies
The Karius test can detect over 1,000 different types of pathogens, including various bacteria, fungi, and DNA viruses, as well as eukaryotic parasites. This broad detection capability makes it useful for diagnosing a range of challenging infections. It can identify pathogens that are difficult to culture using traditional methods, those causing deep-seated infections, or when rapid diagnosis is needed.
Specific conditions where the Karius test can be helpful include endocarditis, an infection of the heart’s inner lining, and osteomyelitis, an infection of the bone. The test is also applied to cases of fevers of unknown origin, especially in immunocompromised patients where traditional tests may be less effective. Prosthetic joint infections and meningitis or encephalitis, infections of the brain and spinal cord membranes, also benefit from this technology due to the difficulty in culturing the causative organisms or accessing the infection site. The test can detect pathogens even when patients have already received antimicrobial therapy, as it identifies cfDNA released from both living and dying microbes.
Why the Karius Test is a Significant Advance
The Karius test offers several advantages over traditional diagnostic methods for infectious diseases. One benefit is its speed, with results typically available the day after sample receipt, which can lead to earlier and more targeted treatment. This contrasts with standard culture-based methods that can take days to weeks to yield results. The test’s ability to provide rapid insights can improve patient management, particularly in cases of severe infections like sepsis.
Another advantage is its broad detection capability, allowing it to identify over 1,000 pathogens simultaneously from a single blood sample. This comprehensive nature helps in detecting organisms that are slow-growing or cannot be cultured using conventional techniques. The Karius test is also non-invasive, requiring only a simple blood draw, which can reduce the need for more invasive procedures like tissue biopsies. This is beneficial for patients who are critically ill or have compromised immune systems, as invasive procedures can carry additional risks.
Factors to Consider with the Karius Test
While the Karius test presents a promising diagnostic tool, several factors warrant consideration. The cost of the test can be a significant factor, with a list price around $2,000, and it may not always be fully covered by insurance. Reimbursement practices vary, and the test is mostly utilized for acutely ill hospitalized patients.
Availability of the Karius test is another consideration, as it is a specialized laboratory-developed test (LDT) performed by Karius, Inc., and is not universally available or adopted in all healthcare settings. While the Karius laboratory is CLIA-certified and CAP-accredited, the test is not required to be cleared or approved by the FDA, though the FDA is exploring regulatory approaches for such technologies.
Clinical interpretation of the results is also important; detecting microbial DNA does not always signify an active infection. Microbial cell-free DNA can be present due to colonization, contamination, or from dead or dying organisms, even after successful treatment. Therefore, clinicians must carefully consider the patient’s overall clinical picture, symptoms, and other diagnostic findings to differentiate between true infection and other scenarios.
The test’s performance characteristics, including sensitivity and specificity, are high, with analytical specificity often reported at over 99.99% per microorganism. However, like any diagnostic tool, false positives or negatives can still occur. It is understood as a diagnostic aid that complements traditional methods rather than a standalone solution, and ongoing research continues to refine its use and explore potential expanded applications.