thebench.bio #1 Ecosystem for the Bio-Tech Industry
All PCR & qPCR questions

Question 19 of 20 Advanced Concept

When would digital PCR be a better choice than qPCR?

Short answer

Digital PCR splits a sample into thousands of partitions and counts the positive ones, giving absolute copy numbers without a standard curve. It wins when you need absolute quantification, precise measurement of small differences, detection of a rare variant in a large wild-type background, or tolerance to inhibitors.

Why

In digital PCR each partition holds zero, one or a few target molecules, a distribution described by Poisson statistics. After end-point amplification the fraction of positive partitions gives the concentration directly. No calibrator is needed, so errors in standard preparation disappear.

This suits reference material value assignment, vector copy number, and viral load where an absolute figure matters. Because each partition is read as on or off, moderate inhibition that delays a qPCR curve often still lets a partition turn positive. Partitioning also concentrates a rare mutant away from wild-type molecules, which helps detect mutations at low fractions, as in liquid biopsy work. qPCR keeps advantages.

It has a wider dynamic range, because digital PCR saturates once nearly every partition is positive. It is cheaper, faster and higher throughput, and it handles larger reaction volumes. For example, checking a 1.2-fold copy number change is a digital PCR job, while screening 300 samples for gene expression across a 10,000-fold range suits qPCR.

Digital PCR still needs care with threshold setting, partition volume and preanalytical steps, as the dMIQE 2020 update stresses.

A strong answer covers

  • Partition counting with Poisson correction gives absolute copies
  • No standard curve needed
  • Better precision for small fold differences and rare alleles
  • More tolerant of inhibitors
  • qPCR wins on dynamic range, throughput and cost

Common mistakes

  • Assuming digital PCR has no error because it counts molecules
  • Loading so much template that nearly every partition is positive and precision collapses
  • Ignoring partition volume when converting positive fraction to concentration