PCR Variations

Multiplex PCR

Multiplex PCR amplifies several targets in one tube with several primer pairs ¹. Respiratory panels detecting a dozen or more viruses, STR identity kits amplifying 20+ loci, and an internal control alongside a target are all multiplexes.

The balancing act:

• All primers must work at one annealing temperature and must not form dimers with each other.
• Targets compete for reagents; an abundant target can suppress a weak one, so primer concentrations are tuned.
• Products are told apart by size (gel or capillary), by dye color (real-time probes in different channels), or by melt temperature or array hybridization.

Each added target multiplies the validation work.

Allele-specific PCR

Allele-specific PCR (ARMS, amplification refractory mutation system) puts the variant base at a primer’s 3’ end ¹. A matched 3’ end extends; a mismatched one doesn’t, because the polymerase needs a paired 3’-OH. Designs often add a deliberate second mismatch a few bases from the 3’ end to sharpen discrimination.

Typical setup: one reaction or primer for each allele, plus a control product to show the reaction worked. Uses include point-mutation genotyping (Factor V Leiden, JAK2 V617F) and detecting a low-level mutation against a large normal background. Proofreading polymerases defeat the design by trimming the mismatched 3’ base, so ARMS uses Taq-type enzymes.

Digital PCR

Digital PCR (dPCR) splits the reaction into thousands of tiny partitions - nanowells, or oil-separated droplets in droplet digital PCR (ddPCR) - so each holds zero, one, or a few target molecules. Every partition is amplified to endpoint and scored positive or negative ¹.

Some positive partitions held more than one molecule, so a plain count undercounts. Molecules scatter randomly, following the Poisson distribution, and the average copies per partition is λ = -ln(1 - p), where p is the fraction positive ¹. Divide by partition volume to get copies per microliter.

What it buys:

• Absolute quantification without a standard curve.
• Precision at low levels and tolerance of inhibitors and efficiency differences, since endpoint scoring doesn’t depend on reaction rate ¹.
• Rare variants: a few mutant copies among many normal ones are each isolated in their own partitions.

Uses: minimal residual disease and BCR::ABL1 deep molecular response, liquid biopsy mutations in cell-free DNA, copy-number measurement, and value-assigning reference materials.

High-throughput formats

Array and microfluidic platforms run hundreds to thousands of nanoliter PCRs side by side, each well preloaded with a different assay, for many targets times many samples at low cost per data point ¹. Pharmacogenomic genotyping panels and expression arrays use these.

What to take from this

Multiplex PCR amplifies several targets at once but needs primers balanced at one annealing temperature and products separated by size or color. Allele-specific PCR exploits the 3’ mismatch rule (with Taq, not proofreading enzymes). Digital PCR partitions the reaction, counts positive partitions, and corrects with λ = -ln(1 - p) for absolute copies with no standard curve - ideal for rare variants and low levels.