What are sequencing metrics
Illumina Sequencing run metrics
Run performance for Illumina sequencing can be monitored using the Illumina native programmes: Sequencing Analysis Viewer (SAV) and BaseSpace. Next, we will discuss the key metrics.
Yield (Gb)
This shows the number of bases generated in the run (Figure 1). Different sequencers and runs generate different amounts of data over variable amounts of time.
Figure 1 – Summary of sequencing metrics from BaseSpace (1. Yield (Gb); 2. % ≥ Q30; 3. % Aligned and % error rate; 4. Cluster Density (K/mm2); 5. %PF).
% ≥ Q30
This is the percentage of bases with a quality score of 30 or higher. It is evaluated using a Q Score distribution chart that will indicate the % of reads with Q Score ≥ 30
- Q30 is the probability of incorrect base calling of 1 in 1000 (99.9% accuracy)
- lllumina runs usually generate > 70-80% Q30 data
- The Q30 reported is an average across the whole read length
- The Q30 decreases towards the end of the read
Percent Aligned and Error Rate
Illumina recommends that all runs are spiked with their PhiX control; these metrics are only calculated if PhiX is spiked in.
- The PhiX library provides a quality control for cluster generation, sequencing, and alignment.
- % Aligned is the percentage of clusters in which the first 25 cycles align to the PhiX reference genome.
- Error rate is the rate of mismatches between sequencing data and the PhiX reference genome (usually below 1%). If this was not used then %>=Q30 is your best tool to check base quality.
Cluster Density (K/mm2)
This shows the density of clusters on the flow cell and is an important metric to evaluate the quality of the data (Figure 1). When looking at these data you should also consider the Percentage Clusters Passing Filter (%PF), which is an internal quality filtering procedure. Together these metrics can help you determine if there are problems with your loading concentration.
Cluster density and %PF are inversely related: typically if there is a high cluster density the %PF will be lower and vice versa:
- Lower %PF may lead to losing coverage on some samples within the run
- If the samples are not inversely related (for example cluster density is within range but %PF is low) consider an instrument or reagent issue
High cluster density: overloading of the library may cause merging of clusters which increases the cluster density. This may lead to:
- Poor template generation, which then causes a decrease in the percentage passing filter (%PF)
- Low Q30 scores
- Complete run failure
Low cluster density can be due to:
- Underloading of the library
- Library preparation issues (for example, poor denaturation, etc.)
Both high and low cluster densities will have a negative impact on data output.
Oxford Nanopore sequencing metrics
MinKnow provides real-time information about the sequencing run. It is important to consider flow cell health, pore occupancy and read length. If live base-calling is active, quality score plots and information of number reads per barcode can also be followed. A report is then generated at the end of the run.
Flow cell health and pore occupancy
The flow cell health graph provides a summary of the status of all the pores in a flow cell (Figure 2). A colour code is used to indicate different pore statuses (Figure 3).
Figure 2 – Sequencing overview showing flow cells currently sequencing on MinKnow software indicating the flow cell health
Figure 3 – Table showing colour-coding of different flow cell pore occupancy statuses.
Pore occupancy information is provided in real-time by the channel states panel (Figure 4). A high proportion of active sequencing channels is an indicator of good library preparation.
Figure 4 – Channel states panel: real-time pore occupancy status of one sequencing run on MinKnow software
The pore activity graph summarises the channel state of a run over time (Figure 5). In a good run, the proportion of active pores decreases slowly as the run progresses.
Figure 5 – Pore activity: Graph summarising pore activity over time on MinKnow software.
Read length histograms and cumulative plots
Read length histograms provide cumulative information on the read length (Figure 6). This information is presented either as number of reads, or, estimated bases vs read length.
Figure 6 – Read length histogram: Graph showing estimated read length vs estimated bases on MinKnow software.
Cumulative output graphs show the total number of reads and if they have passed or failed the quality filter (Figure 7). If live base-calling is not active, only the total number of reads will be displayed.
Figure 7 – Cumulative output: Graph showing the total number of reads and whether they have passed or failed the quality filter on MinKnow software
If live base-calling is active, cumulative median or modal quality score plots of passed reads are available (Figure 8).
Figure 8 – Quality score: Graph showing quality score over time on MinKnow software
Barcode stats
If multiple samples are being sequenced, the barcode read counts graph displays the number of reads that have been basecalled and have passed the quality filter for each barcode (Figure 9).
Figure 9 – Barcode hits: Graph showing the read counts for each barcode on MinKnow software.
A Practical Guide for SARS-CoV-2 Whole Genome Sequencing
A Practical Guide for SARS-CoV-2 Whole Genome Sequencing
