Introduction:
- A molecular test is a type of medical test that looks for the presence or absence of specific genetic material in a sample taken from the body. These tests are often used to diagnose infectious diseases, such as COVID-19, and to determine a person’s risk for certain inherited conditions.
- Molecular tests use techniques such as Polymerase Chain Reaction (PCR) or nucleic acid amplification to detect the presence of specific DNA or RNA sequences in a sample. They are highly sensitive and specific, and can often detect small amounts of genetic material that may not be detectable by other types of tests.
- Molecular tests are often used to diagnose infectious diseases because they can quickly and accurately identify the presence of a specific virus or bacterium. They are also used to monitor the effectiveness of treatments for certain conditions, such as cancer, and to screen for inherited genetic conditions.
Principle:
- The principle of molecular tests is based on the detection and identification of specific genetic material, such as DNA or RNA, in a sample taken from the body. Molecular tests use techniques such as polymerase chain reaction (PCR) or nucleic acid amplification to amplify (copy) specific regions of DNA or RNA many times, making them easier to detect.
- The amplified DNA or RNA is then detected using techniques such as fluorescence or colorimetry, which can identify the presence of specific sequences. Other techniques, such as hybridization and sequencing, can be used to detect or identify specific DNA or RNA sequences in a sample.
- Overall, the specific mechanism of a molecular test depends on the specific test being performed and the type of genetic material being detected.
Concept of molecular diagnostics:
- Molecular diagnostics is the use of molecular techniques, such as PCR and nucleic acid amplification, to diagnose diseases and conditions. These techniques allow for the detection and identification of specific genetic material, such as DNA or RNA, in a sample taken from the body.
- Molecular diagnostics has revolutionized the field of medicine by allowing for the rapid and accurate diagnosis of a wide range of diseases and conditions. These tests are highly sensitive and specific, and can often detect small amounts of genetic material that may not be detectable by other types of tests.
- Molecular diagnostics is commonly used to diagnose infectious diseases, such as COVID-19, and to screen for inherited genetic conditions. It is also used to monitor the effectiveness of treatments for certain conditions, such as cancer, and to determine a person’s risk for certain inherited conditions.
Specimens:
The specific type of sample used for molecular testing depends on the specific test being performed and the type of genetic material being detected. There are many different types of samples that can be used for molecular testing, including:
Blood
Blood is a common sample type used for molecular testing, as it contains DNA and RNA from various cells in the body. Blood samples can be collected using a needle and a vial, or using a fingerstick or heel stick for smaller amounts of blood.
Saliva
Saliva samples can be collected using a saliva collection kit or by having a person spit into a tube or cup. Saliva contains DNA and RNA from cells in the mouth, and can be used for a variety of molecular tests.
Nasal or throat swabs
Nasal or throat swabs can be used to collect samples from the respiratory tract for molecular testing. These swabs are inserted into the nostril or throat and then rotated to collect cells and mucus.
Fig: Specimens used in molecular Diagnostics Test
Urine
Urine samples can be collected for molecular testing, although they are less commonly used than other types of samples. Urine contains DNA and RNA from cells in the urinary tract, and can be used to diagnose infections or other conditions.
Tissue
Tissue samples, such as biopsy samples or surgical specimens, can be collected for molecular testing. These samples contain DNA and RNA from specific types of cells and can be used to diagnose cancer or other conditions.
Bacteria
The bacterial pathogens which are unidentifiable with routine testing, slow growers, uncultivable can be identified by molecular testing.
Amniotic fluid
A sample of amniotic fluid, which contains fetal cells and chemicals produced by the baby. The extracted DNA from the cells is used for diagnosis of certain conditions, such as Down syndrome.
Types of Molecular Tests:
There are many different types of molecular tests that are used for a variety of purposes. Some common types of molecular tests include:
Polymerase chain reaction (PCR) tests: These tests amplify specific regions of DNA or RNA in a sample using enzymes called polymerases and specific primers that bind to the target sequence. PCR tests are commonly used to diagnose infectious diseases, such as COVID-19, and to monitor the effectiveness of cancer treatments.
Ligase chain reaction (LCR) tests: These tests amplify specific DNA or RNA sequences using ligase enzymes, which join together specific DNA sequences. LCR tests are often used to diagnose infectious diseases and to screen for inherited genetic conditions.
Hybridization tests: These tests use probes, which are specific pieces of DNA or RNA that bind to target sequences, to detect the presence of specific genetic material in a sample. Hybridization tests are commonly used to diagnose infectious diseases and to screen for inherited genetic conditions.
Sequencing tests: These tests determine the specific order of nucleotides in a DNA or RNA molecule. Sequencing tests are used to identify genetic mutations and to study the genetic basis of diseases and conditions.
Microarray tests: These tests use a specialized chip to detect the presence of specific DNA or RNA sequences in a sample. Microarray tests are often used to detect genetic mutations and to study gene expression.
Next-generation sequencing (NGS) tests: These tests use high-throughput sequencing techniques to rapidly sequence large amounts of DNA or RNA. NGS tests are used for a wide range of applications, including the diagnosis of genetic conditions and the study of genetic variation.
Instruments used in molecular Tests:
There are many different instruments and equipment used in molecular testing. The special instruments include are:
Polymerase chain reaction (PCR) machines (Thermocycler): These machines are used to amplify specific regions of DNA or RNA using enzymes called polymerases and specific primers that bind to the target sequence. PCR machines typically have a thermal cycler that rapidly heats and cools the sample to facilitate the amplification process.
Nucleic acid amplification machines: These machines use enzymes to amplify specific DNA or RNA sequences. Examples include ligase chain reaction (LCR) machines and transcription-mediated amplification (TMA) machines.
Hybridization equipment: These instruments are used to detect the presence of specific DNA or RNA sequences using probes that bind to the target sequence. Examples include Southern blots, Western blots, Northern blots, and dot blots.
Sequencing instruments: These instruments are used to determine the specific order of nucleotides in a DNA or RNA molecule. Examples include capillary sequencers and next-generation sequencing (NGS) machines.
Fig: Instruments used in Molecular Diagnostics Tests
Microarray equipment: These instruments use a specialized chip to detect the presence of specific DNA or RNA sequences in a sample. Examples include DNA microarrays and RNA microarrays.
Other equipment: Other equipment used in molecular testing includes pipettes, centrifuges, and specialized reagents and consumables.
In general, the specific instruments and equipment used in molecular testing depend on the specific test being performed and the type of genetic material being detected.
Applications:
Molecular tests have a wide range of applications in medicine, including the diagnosis of infectious diseases, the screening for inherited genetic conditions, the monitoring of treatment effectiveness, and the study of genetic variation. Some specific examples of the application of molecular tests include:
Diagnosis of infectious diseases: Molecular tests are commonly used to diagnose infectious diseases, such as COVID-19, influenza, and HIV. These tests can quickly and accurately identify the presence of specific viruses or bacteria in a sample, allowing for prompt treatment and disease control measures.
Screening for inherited genetic conditions: Molecular tests can be used to screen for inherited genetic conditions, such as cystic fibrosis, sickle cell anemia, and hereditary breast and ovarian cancer. These tests can help identify individuals who are at risk for these conditions and allow for earlier diagnosis and treatment.
Monitoring treatment effectiveness: Molecular tests can be used to monitor the effectiveness of treatments for certain conditions, such as cancer. These tests can detect the presence of specific genetic markers in a sample, indicating the presence of cancer cells or the effectiveness of treatment.
Studying genetic variation: Molecular tests can be used to study genetic variation within and between populations. This can provide important insights into the genetic basis of diseases and conditions, and can help inform the development of new treatments and therapies.
References:
- Amos, J., Patnaik, M., 2002, Commercial molecular diagnostics in the U.S.: The Human Genome Project to the clinical laboratory. Hum Mutat 19:324-333.
- Dennis, Lo, Y.M., Wittwer, C.T., 2009, Molecular Diagnostics: At the Cutting Edge of
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- Choe H, Deirmengian CA, Hickok NJ, Morrison TN, Tuan RS. Molecular diagnostics. J Am Acad Orthop Surg. 2015 Apr;23 Suppl(0):S26-31