Using PCR to Detect Viruses in Blood and Tissue Samples

A 1989 patent by Nobel laureate Kary Mullis and his team on using polymerase chain reaction (PCR) to replicate and detect tiny amounts of viral DNA or RNA, such as HIV and Hepatitis B, directly from human clinical samples.

What it covers

This patent describes a method to detect viruses in human samples like blood or semen by copying their genetic material millions of times using Polymerase Chain Reaction (PCR). The process starts by mixing the sample with two short DNA starters called primers, free genetic building blocks (nucleoside triphosphates), and a copying enzyme (polymerase). First, the primers bind to the viral DNA if it is present. Next, the enzyme builds a copy of the target viral sequence. The mixture is then heated to separate the newly made strands, and the cooling and copying cycle is repeated multiple times to amplify the target sequence to detectable levels. Finally, a labeled probe binds to the amplified DNA to confirm the virus is present. For example, this method can detect HIV or Hepatitis B in a patient's blood sample without needing to grow the virus in a laboratory first.

What it doesn't cover

• —Does not cover detecting viruses using antibody-based tests (like ELISA) that do not amplify nucleic acids.
• —Does not cover amplification techniques that do not use thermal denaturation to separate DNA strands (such as isothermal amplification).
• —Does not cover genetic amplification of non-viral DNA, such as human genomic DNA or bacterial DNA.
• —Does not cover methods that detect viral proteins rather than viral nucleic acids (DNA or RNA).

The clever bit

Instead of trying to find a needle in a haystack, this method copies the needle millions of times until it is the only thing you see. It combined PCR amplification with specific hybridization probes to ensure that even a single viral particle in a complex biological sample could be reliably identified.

Why it matters

This patent laid the foundation for modern molecular diagnostics, enabling fast and highly sensitive testing for infectious diseases. Before this, detecting viruses like HIV required slow, difficult, and sometimes dangerous viral cultures. By allowing direct detection of viral genetic material in blood or semen, it revolutionized blood donor screening, clinical diagnostics, and viral load monitoring.

Real-world examples

• 1.Qualitative PCR tests for HIV-1 and Hepatitis B
• 2.Blood bank screening assays for viral contamination
• 3.Early infant diagnosis of HIV using dried blood spots
• 4.RT-PCR diagnostic kits for viral detection