Understanding Viral Testing: PCR Tests and Immunoassays

When a new virus emerges, understanding who is infected is crucial for containment. Two primary testing methods play a vital role: PCR tests and immunoassays. These techniques help determine if someone currently has the virus or if they've previously been infected. Let's delve into the science behind these essential tests.

PCR Tests: Detecting Active Infections

PCR, or polymerase chain reaction, tests are designed to detect the virus's genetic material within the body. They are used to diagnose active infections. Because the amount of viral genetic material might be minuscule, PCR amplifies it to readily observable quantities.

How PCR Works

1. Sequencing the Genome: Researchers sequence the virus's genetic material (genome) to identify unique regions specific to that virus.
2. Targeting Specific Segments: PCR tests target these unique segments.
3. Sample Collection: Samples are collected (e.g., blood for hepatitis, feces for poliovirus, nasal/throat swabs for coronaviruses).
4. Laboratory Analysis: The sample is sent to a lab where PCR is performed to detect the virus's genome.
5. Reverse Transcription (if needed): If the virus uses RNA (like SARS-CoV-2), it must be reverse transcribed into complementary DNA before PCR.
6. Amplification and Detection: If the virus is present, complementary primers identify and copy its unique genetic code. One strand of DNA becomes hundreds of millions, detected using fluorescent dye-marked probes.
7. Positive Result: Fluorescence indicates a positive result, meaning the individual is currently infected.

Immunoassays: Revealing Past Infections

Immunoassays tap into the immune system's memory of a virus, revealing if someone has been infected in the past. They target virus-specific antibodies produced by the immune system during infection. These antibodies are specialized proteins that identify and fight foreign substances like viruses.

Types of Antibodies Detected

• IgM Antibodies: These are the first antibodies produced in response to a new infection. Their presence suggests a recent infection, but they are unreliable for diagnosing current infections because it takes over a week for the body to produce detectable amounts.
• IgG Antibodies: These antibodies circulate for an extended period after infection. Their presence usually indicates past exposure and recovery.

How Immunoassays Work

1. Blood Sample Collection: A blood sample is drawn from the individual.
2. Viral Contact: The sample comes into contact with a portion of the virus of interest.
3. Antibody Binding: If the body has been exposed to the virus, virus-specific antibodies will bind to it.
4. Color Change: This reaction produces a color change, indicating a positive result and past exposure to the virus.

Immunoassays are particularly useful for retroactively diagnosing individuals who were infected but never tested. Furthermore, plasma from individuals with immunity holds potential as a treatment for those currently fighting the virus.

Advancements in Viral Testing

Both PCR and immunoassays are continuously evolving to become more accurate and efficient.

• Digital PCR: This innovation quantifies individual pieces of target DNA, promising increased accuracy.
• Rapid Development: Researchers have demonstrated the ability to quickly develop immunoassays, even before a pandemic is declared.

These tests, along with the scientists and healthcare professionals who develop and administer them, are essential for saving lives, especially when deployed early in an outbreak.