version: "1.0.1" name: bio-read-qc-quality-filtering description: Filters reads by quality, length, N content, and complexity with Trimmomatic, fastp, and Cutadapt, including sliding-window trimming, per-read unqualified-base filtering, and 2-color poly-G removal. Use when reads have poor-quality tails, when an assembly or k-mer workflow needs clean input, or when a junk read subpopulation must be dropped. For adapter removal use adapter-trimming; for all-in-one preprocessing use fastp-workflow. tool_type: cli primary_tool: trimmomatic

Version Compatibility

Reference examples tested with: Trimmomatic 0.39+, fastp 0.23+, Cutadapt 4.4+

Before using code patterns, verify installed versions match. If versions differ:

• CLI: <tool> --version then <tool> --help to confirm flags

If code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.

Quality Filtering -- trim lightly or not at all, and never without a length filter

Trim low-quality bases and drop low-quality reads with Trimmomatic (sliding window / MAXINFO), fastp (per-read filter + window cut), or Cutadapt (BWA-style quality trim).

"Filter reads by quality" -> Remove low-quality bases and/or discard reads below quality/length thresholds.

• CLI: fastp -i in.fq -o out.fq --cut_right -q 20 -l 36 (window trim + per-read filter + length gate)
• CLI: trimmomatic SE in.fq out.fq SLIDINGWINDOW:4:20 MINLEN:36

Scope: this skill OWNS quality/length/N/complexity filtering. Adapter removal -> read-qc/adapter-trimming. Single-pass trim+QC -> read-qc/fastp-workflow. Reading the quality plots -> read-qc/quality-reports. OUT OF SCOPE: contamination removal (read-qc/contamination-screening).

The Single Most Important Modern Insight

1. Modern local aligners SOFT-CLIP low-quality tails, so quality trimming is usually unnecessary -- and AGGRESSIVE quality trimming actively harms downstream results. Williams 2016 showed aggressive trimming changed expression estimates for >10% of genes; Del Fabbro 2013 showed stringent Q>30 DEGRADES de novo assembly; MacManes 2014 found gentle trimming (remove only Phred<2-5) optimal for RNA-seq; GATK discourages quality trimming because BQSR recalibrates qualities itself. Trim ADAPTER always (read-qc/adapter-trimming); quality-trim lightly or not at all before a soft-clipping aligner. The workflows that genuinely need quality trimming are assembly, k-mer/pseudo-alignment, small-RNA, amplicon, and variant calling WITHOUT BQSR.

1. Quality FILTERING (drop whole reads) and quality TRIMMING (cut bases within a read) are different operations with different tools. fastp's -q/-u/-n filters whole reads by the fraction of unqualified bases; --cut_right / Trimmomatic SLIDINGWINDOW trims bases from a window scan. Filtering removes a junk subpopulation (a low-Q hump in the per-sequence-quality plot); trimming shortens reads with decayed tails. Choose by whether the problem is some bad reads or bad ends.

1. A short post-trim read mis-maps, so quality trimming MUST be paired with a minimum-length filter. Williams 2016 showed that adding a post-trim min-length filter mitigates most of the expression distortion that trimming introduces, because over-trimmed fragments that would map spuriously are dropped instead. MINLEN (Trimmomatic, always last), -l (fastp), -m (cutadapt) are not optional add-ons; they are the safety mechanism that makes trimming safe.

Two-color note: on NextSeq/NovaSeq the quality scores are binned to four values (RTA3: 2, 12, 23, 37), so a sliding-window threshold like 4:15 partitions between the 12 and 23 bins rather than acting on a smooth gradient -- thresholds tuned on HiSeq-era 0-40 qualities behave differently. And poly-G tails are HIGH quality, so a quality filter does not remove them (use poly-G trimming).

Tool Taxonomy

Decision Tree by Scenario

Default when uncertain: trim adapter, apply a light window trim plus a minimum-length filter, then confirm with FastQC.

Trimmomatic

Steps run in COMMAND-LINE ORDER; put quality steps before MINLEN so the length check reflects all trimming.

# Single-end: light leading/trailing + window, length-gated
trimmomatic SE -phred33 in.fq.gz out.fq.gz \
    LEADING:3 TRAILING:3 SLIDINGWINDOW:4:20 MINLEN:36
 
# Paired-end (four outputs: paired + orphan)
trimmomatic PE -phred33 -threads 8 \
    R1.fq.gz R2.fq.gz \
    R1_paired.fq.gz R1_unpaired.fq.gz R2_paired.fq.gz R2_unpaired.fq.gz \
    SLIDINGWINDOW:4:20 MINLEN:36
 
# MAXINFO: adaptive length-vs-quality balance (strictness <0.2 favors length, >0.8 favors correctness)
trimmomatic SE in.fq.gz out.fq.gz MAXINFO:40:0.5 MINLEN:36

Do NOT HEADCROP the first ~12 bp of RNA-seq to "fix" the wavy per-base-content plot: that pattern is random-hexamer priming bias (Hansen 2010), not adapter, and trimming it just discards real data without removing the underlying bias.

fastp

fastp separates per-read FILTERING from window TRIMMING. Quality filtering is on by default (-q 15).

# Per-read quality filter: base < Q20 is 'unqualified'; drop read if >40% unqualified or >5 Ns
fastp -i in.fq.gz -o out.fq.gz -q 20 -u 40 -n 5 -l 36
 
# Window trim from the 3' (Trimmomatic SLIDINGWINDOW analogue) + length gate
fastp -i R1.fq.gz -I R2.fq.gz -o R1.fq.gz -O R2.fq.gz \
      --cut_right --cut_window_size 4 --cut_mean_quality 20 -l 36
 
# Drop globally low-quality reads by mean quality (filter, not trim)
fastp -i in.fq.gz -o out.fq.gz -e 25
 
# 2-color poly-G (auto-enabled for NextSeq/NovaSeq from the instrument ID)
fastp -i in.fq.gz -o out.fq.gz --trim_poly_g
 
# Low-complexity filter (e.g. poly-A / homopolymer-rich reads)
fastp -i in.fq.gz -o out.fq.gz --low_complexity_filter --complexity_threshold 30

fastp flags: -q qualified quality (default 15), -u unqualified percent limit (default 40), -n N base limit (default 5), -e average-quality filter (default 0 = off), -l length required (default 15), --length_limit max length (long form only), --cut_front/--cut_tail/--cut_right window cut modes (off by default), --cut_window_size (4), --cut_mean_quality (Q20).

Cutadapt

-q uses the BWA running-partial-sum algorithm, not a fixed cutoff, so a single high-Q base inside a low-Q run does not stop trimming. Quality trimming runs BEFORE adapter removal.

# 3'-only quality trim with a length gate (5',3' form: -q 15,20)
cutadapt -q 20 -m 36 -o out.fq.gz in.fq.gz
 
# Combined adapter + light quality trim, paired
cutadapt -a AGATCGGAAGAGC -A AGATCGGAAGAGC -q 20 -m 36 \
         -o R1.fq.gz -p R2.fq.gz R1.fq.gz R2.fq.gz

Quantitative Thresholds

Common Errors

References

Bolger AM, Lohse M, Usadel B. 2014. Trimmomatic: a flexible trimmer for Illumina sequence data. Bioinformatics 30(15):2114-2120. Chen S, Zhou Y, Chen Y, Gu J. 2018. fastp: an ultra-fast all-in-one FASTQ preprocessor. Bioinformatics 34(17):i884-i890. MacManes MD. 2014. On the optimal trimming of high-throughput mRNA sequence data. Frontiers in Genetics 5:13. Del Fabbro C, Scalabrin S, Morgante M, Giorgi FM. 2013. An extensive evaluation of read trimming effects on Illumina NGS data analysis. PLoS ONE 8(12):e85024. Williams CR, Baccarella A, Parrish JZ, Kim CC. 2016. Trimming of sequence reads alters RNA-Seq gene expression estimates. BMC Bioinformatics 17:103. Hansen KD, Brenner SE, Dudoit S. 2010. Biases in Illumina transcriptome sequencing caused by random hexamer priming. Nucleic Acids Research 38(12):e131.

Related Skills

read-qc/adapter-trimming - Remove adapter before quality filtering read-qc/quality-reports - Read the quality plots that motivate filtering read-qc/fastp-workflow - All-in-one preprocessing in a single pass read-alignment/bwa-alignment - Soft-clipping aligner that absorbs low-quality tails read-alignment/star-alignment - Soft-clipping RNA aligner (light trimming preferred)