Bio-Rad’s QX100 Droplet Digital™ PCR System Provides Accurate Quantification of Next-Generation Sequencing Libraries

A study published today found that Droplet Digital PCR (ddPCR™) can be used as an accurate and precise method for quality control of next-generation sequencing (NGS) libraries. NGS library QC is essential to optimizing sequencing data yield, thereby increasing efficiency and throughput while lowering cost.

"While real-time PCR has traditionally been used to quantify libraries, we determined that the only truly accurate way to reproducibly quantify our NGS libraries is with ddPCR," said Dr. Jason Bielas, lead author and Assistant Member in the Public Health Sciences Division at Fred Hutchinson Cancer Research Center in Seattle, Wash.

Quantifying NGS Libraries and Why It Matters
Various commercial NGS technologies require users to load a precise number of viable DNA library molecules onto the instrument to optimize data yield. Performing a sequencing run with either too many or too few library molecules results in compromised data and sometimes no data at all – wasting sample, expensive reagents, user time, and instrument time.

Moreover, fewer bases might be sequenced if library molecules are not the appropriate length to fully utilize the sequencing platform, thus limiting throughput. Given this, quantifying library molecules and determining fragment size range have become crucial steps in library preparation.

NGS instrument manufacturers recommend quantifying libraries using real-time quantitative PCR (qPCR) and determining their size range using gel or capillary electrophoresis. Each of these has its limitations, though, and the steps recommended to address them, can be time-consuming and expensive.

Advantages of ddPCR for Quantifying NGS Libraries
To simultaneously quantify and determine the size distribution of target DNA with a single ddPCR assay, Dr. Bielas and his team exploited a relationship between droplet fluorescence and amplicon size. They confirmed the accuracy and precision of this method by applying it to NGS library preparation.

The ddPCR assay they designed – known as QuantiSize – was developed using the QX100 ddPCR system from Bio-Rad Laboratories. QuantiSize offers the ability to determine the absolute quantity and the detailed size distribution of target DNA in a single ddPCR reaction well, thus avoiding the drawbacks of other independent quantification and size determination methods.

"Now that we have discovered this new correlation, we can also use ddPCR to extract more information on the characteristics of DNA based on the range of fluorescence that can occur within each droplet," said Bielas.

Having demonstrated the efficacy of this technique, Dr. Bielas is now planning to leverage the relationship between ddPCR fluorescence and amplicon size to explore mutagenic deletion events in both the human nuclear and mitochondrial genomes.