Multiplex

Multiplex PCR & Compatibility

Design and validate primers for Multiplex PCR, where multiple targets are amplified in a single reaction.

Overview

Multiplex PCR saves time and reagents but requires careful primer design to prevent:

• Primer Dimers: Primers binding to each other.
• Cross-Reactivity: Primers amplifying off-target regions.
• Competition: One target amplifying much more efficiently than others.

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Workflow

1. Design Individual Pairs

First, design primers for each target independently. Crucial: Use the same melting temperature (Tm) settings for all targets.

# shared_params.yaml - Use for all targets
parameters:
  tm:
    min: 59.0
    opt: 60.0
    max: 61.0  # Narrow range is best for multiplexing
  gc:
    min: 40.0
    max: 60.0

Run the design for each target:

primerlab run pcr --config shared_params.yaml --sequence target1.fasta --out target1/
primerlab run pcr --config shared_params.yaml --sequence target2.fasta --out target2/

2. Check Compatibility

Use the check-compat command to analyze multiple primer files together.

primerlab check-compat \
  -p target1/primers.json \
  -p target2/primers.json \
  --multiplex

3. Analyze Results

The tool calculates a Compatibility Score based on:

• Self-Dimers: Primers binding to themselves.
• Hetero-Dimers: Forward from Pair A binding to Reverse from Pair B.
• 3’ End Complementarity: Analyzing the critical 3’ region for extension.

Scoring Guide:

• 90-100: Excellent (No significant interactions)
• 80-89: Good (Minor interactions, likely negligible)
• < 80: Risky (High probability of dimers, redesign recommended)

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Troubleshooting Multiplex Reactions

Issue	Solution
Uneven Amplification	Adjust primer concentrations (increase for weak products). Ensure amplicon sizes are different enough to separate on gel/CE.
Missing Bands	Check for primer dimers consuming reagents. Redesign the failing set.
Non-Specific Bands	Increase annealing temperature or perform a “Species Specificity” check.

Feature: Dimer Matrix

For complex multiplex sets (>4 pairs), generate a visual matrix:

primerlab check-compat --primers all_primers.json --plot-matrix dimer_matrix.png

This creates a heatmap showing interaction strength (Delta G) between every combination of primers.