Overview of Quality Control in Sequencing Data

Quality control (QC) in sequencing data refers to a series of processes and evaluations designed to assess the accuracy, reliability, and usability of raw data generated by high-throughput sequencing technologies such as Illumina, Oxford Nanopore, or PacBio.

Why Quality Control Is Crucial for Sequencing Data

  1. Error Detection and Correction
    QC helps identify and filter out errors such as base miscalls, adapter contamination, and poor-quality reads, ensuring the integrity of downstream analyses.

  2. Improved Data Reliability
    By enforcing quality thresholds (e.g., PHRED score > Q30), QC enhances the trustworthiness of biological conclusions drawn from the data.

  3. Reduction of Bias
    Detects and minimizes technical biases such as GC-content imbalance, PCR duplicates, and coverage variation, which can skew results.

  4. Efficient Use of Resources
    Early QC prevents unnecessary computational processing and storage of unusable or poor-quality data.

  5. Compliance and Reproducibility
    QC supports reproducibility and is often required by journals, data repositories, and regulatory standards when publishing or sharing data.

Common QC Steps in Sequencing Workflows

  • Raw Read Quality Assessment
    Use tools like FastQC to evaluate base quality scores, sequence length distributions, GC content, and overrepresented sequences.

  • Adapter Trimming and Filtering
    Tools like Trimmomatic or Cutadapt remove adapter sequences and trim low-quality bases from reads.

  • Duplicate Removal
    Identify and remove PCR duplicates, especially important in DNA sequencing.

  • Contamination Check
    Use tools such as Kraken, Bowtie2, or Decontam to detect and remove unwanted or contaminant sequences.

  • Post-QC Reassessment
    Re-run quality metrics on cleaned data to confirm improvements and ensure readiness for downstream analysis.

Common QC Steps in Sequencing Workflows

           +------------------+
           |  Raw Sequencing  |
           |      Reads       |
           +--------+---------+
                    |
                    v
       +------------+-------------+
       |  Raw Read Quality Check  |  <-- FastQC
       +------------+-------------+
                    |
                    v
      +-------------+--------------+
      | Adapter/Quality Trimming  |  <-- Cutadapt, Trimmomatic
      +-------------+--------------+
                    |
                    v
        +-----------+------------+
        |   Duplicate Removal    |  <-- MarkDuplicates, Picard
        +-----------+------------+
                    |
                    v
        +-----------+------------+
        | Contamination Filtering|  <-- Kraken, Decontam
        +-----------+------------+
                    |
                    v
        +-----------+------------+
        |  Post-QC Quality Check |  <-- FastQC again
        +------------------------+