Bloodstream Infection Testing: Improving the Diagnosis of Bloodstream Infections With Molecular Assays
A number of molecular methods have been developed for detecting the pathogens in whole blood. SeptiFast and SeptiTest are some of the commercially available molecular assays for the diagnosis of bloodstream infections. The increasing number of sepsis cases and growing occurrence of infectious diseases among people around the world are the major factors driving the growth of the bloodstream infection testing market.
Bloodstream infection (BSI) is a life-threatening condition resulting from the presence of microorganisms, generally fungi or bacteria, in the blood. The detection of microorganisms is essential in managing patients and is the major cause of death in developed countries. Bloodstream infections represent a growing concern for public health, with an estimated burden of 1,200,000 cases of BSI each year in Europe. It ranks among the top seven causes of death in Europe and North America. The growing occurrence of infectious diseases and the increasing number of sepsis cases among people around the world are the major factors driving the growth of the bloodstream infection testing market. By 2027, the global bloodstream infection testing market is expected to surpass $7,722.2 million, increasing from $4,065.5 million in 2019.
Commercially Available Molecular Assays for the Diagnosis of BSIs from Whole Blood
Over the years, several molecular methods have been developed for detecting the pathogens in whole blood. The first assays were developed and designed for the detection of a single pathogen of interest. A single-pathogen approach is not useful for the diagnosis of BSIs, as these infections may be caused by a wide range of microorganisms. However, this limitation has been overcome in a number of commercial assays which are able to detect various microorganisms. Some of the commercially available molecular assays for the diagnosis of bloodstream infections from whole blood are as follows:
SeptiFast is a multiplex real-time polymerase chain reaction (PCR) assay that detects 25 pathogens, such as Aspergillus fumigatus and five Candida species. The presence of the meCA resistance gene may be detected with a separate test. The volume of blood required is 1.5 mL or 3 mL initially. The region amplified in this assay is the ITS (internal transcribed spacer) region, which is located between the 18S and 5.8S ribosomal genes for fungi and 16S and 23S ribosomal genes for bacteria. The amplification is performed with the help of a LightCycler 2.0 instrument and specific fluorescent probes are helped to detect different pathogens. Using this approach, the time-to-result is 4.5–6 hours. This assay has been broadly evaluated in the clinical setting, however, the results are contradictory with the reported sensitivities ranging between 15%-98% in ICU patients.
The SepsiTest assay is based on a wide-range PCR amplification followed by sequencing. In this test two 1 mL aliquots of blood are processed in duplicate & human DNA is selectively degraded before the bacterial cell lysis step. Several studies using this approach for the diagnosis of sepsis have been published. The largest study reported a specificity and sensitivity of 85.8% and 87.0%.
A multiplexed PCR analysis, the VYOO assay detects 34 pathogens, including Aspergillus fumigatus and six species of Candida, as well as several resistance genes, such as methicillin resistance gene (mecA), β-lactamase genes blaCTX-M and blaSHV, and vancomycin resistance genes vanA and vanB. The amplified products are envisaged using a conventional gel electrophoresis, and the time-to-result is about 8 hours. For VYOO assay, microbial DNA from 5 mL of blood is enriched and the total DNA is applied to an affinity chromatographic column, which specifically binds the microbial DNA (LOOXTER). Besides, human DNA is worn-out during the extraction step and the sensitivity of this assay ranges from 38.0% to 60.0%.
- Magicplex Sepsis Real-Time Test
Three PCR reactions are necessary in the Magicplex Sepsis assay to achieve the identification at the pathogen’s species level. Firstly, a conventional PCR amplification step is performed, wherein primers are designed to amplify genomic material from 91 microorganisms and three resistance genes are used. Secondly, a real-time PCR is carried out in a screening step to identify the genera level or group of pathogens present. Lastly, a second real-time PCR is performed in order to achieve the identification at species level. With Magicplex Sepsis real-time test assay, the identification of 21 bacterial species, Aspergillus fumigatus, and five Candida species is possible. Human DNA is removed and 1 mL of whole blood is used for the DNA extraction, prior to the lysis of microorganisms. 6 hours is the time-to-result of this assay. The sensitivity and specificity of this assay ranges from 65.0% and 92.0%, respectively.
The Bottom Line
Bloodstream infections are frequent & heterogeneous, and thus may occur in community & nosocomial infections from various sources in different types of patients. These infections are also a major cause of mortality and morbidity in patients admitted to hospital. Bloodstream infection testing is an in-vitro diagnostic technique that is used for detecting the presence of foreign bodies, such as yeast, bacteria, and/or fungus in the blood of an individual. Although major progress has been made in improving the identification of microorganisms in the last decade, blood culture remains the first line tool, and gold standard for diagnosing bloodstream infections and the identification of the responsible pathogens.
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