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Module 4: Quality Control of Raw Reads: Phred Scores, FastQC & MultiQC Diagnostics

1 min read • Verified October 2026 • Peer-Reviewed Learning Resource
Tested Stacks: GATK 4.6.1 Samtools 1.21 Nextflow 24.10 Seurat v5.1 Python 3.12
Prerequisites: Basic Linux CLI & Sequencing Formats
Module 04 • Quality Control Track 18 • Practical Transcriptomics Hands-On Code Included

Module 4: Quality Control of Raw Reads: Phred Scores, FastQC & MultiQC Diagnostics

Deconstructing the 4-line FASTQ structure, mathematical Phred quality scores Q = -10 log10(P), interpreting FastQC diagnostic plots (per-base quality, GC bias, adapter contamination), and aggregating reports with MultiQC.

4. Quality Control of Raw Reads: Phred Scores, FastQC & MultiQC

Each sequencing read is stored in standard 4-line FASTQ format. Line 4 contains ASCII characters representing the base call error probability:

@SRR12345678.1 1/1 ← Header (Instrument ID, Flowcell, Tile, Coordinates, Read 1)
NATCGATCGATCGATCGATCGATC... ← Sequence
+ ← Separator
#<<AFFFFJJJJJJJJJJJJJJJJ... ← ASCII Phred Quality Scores (Sanger / Phred+33)

The Phred Quality Score Formula

A Phred quality score ($Q$) is logarithmically linked to the probability ($P$) that the corresponding base call is incorrect:

$$Q = -10 \times \log_{10}(P)$$ $$\text{Q20} \implies P = 0.01 \quad (99\% \text{ Base Call Accuracy})$$ $$\text{Q30} \implies P = 0.001 \quad (99.9\% \text{ Base Call Accuracy})$$

Running FastQC & Aggregating Reports with MultiQC

# Run FastQC on all raw paired FASTQ files using 8 CPU threads
fastqc -t 8 00_raw_data/*.fastq.gz -o 01_qc_raw/

# Aggregate all individual HTML reports into one interactive MultiQC dashboard
multiqc 01_qc_raw/ -o 01_qc_raw/ --interactive --title "Raw Reads Quality Control"
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Common Pitfalls & Troubleshooting Checklist

  • Java Heap Exhaustion (GATK / Picard): If processes terminate with java.lang.OutOfMemoryError, set your environment variable explicitly: export _JAVA_OPTIONS="-Xmx16g -Djava.io.tmpdir=/tmp".
  • Index Timestamp Mismatch: If tools complain that a BAM index is missing or out of date, regenerate with samtools index -@ 4 sample.bam.
  • Conda Channel Conflicts: Avoid ABI incompatibilities between bioconda and conda-forge by setting strict priority: conda config --set channel_priority strict.
  • Chromosome Naming Contamination: Always verify whether reference FASTA headers use chr1 or 1; divergent naming causes silent alignment drops.
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