> ## Documentation Index
> Fetch the complete documentation index at: https://primerlab-genomic.mintlify.site/llms.txt
> Use this file to discover all available pages before exploring further.

# Understanding results

# Understanding Results

A comprehensive guide to interpreting PrimerLab output, scores, warnings, and recommendations.

## Quality Scores (0-100)

Every primer pair receives a quality score based on multiple factors. The score starts at 100 and penalties are applied for sub-optimal characteristics.

### Score Categories

| Score Range |    Grade    | Meaning                        | Action                    |
| :---------: | :---------: | ------------------------------ | ------------------------- |
|    85-100   | ✅ Excellent | Optimal design, minimal issues | Use with confidence       |
|    70-84    |    ✅ Good   | Acceptable, minor issues       | Generally safe to use     |
|    50-69    |   ⚠️ Fair   | Notable issues detected        | Review warnings carefully |
|     0-49    |    ❌ Poor   | Significant problems           | Consider redesign         |

***

## Penalty Breakdown

PrimerLab applies penalties for the following issues:

| Issue                | Penalty (Standard) | What It Means                                  |
| -------------------- | :----------------: | ---------------------------------------------- |
| **Primer3 Penalty**  |      -0 to -50     | Base penalty from thermodynamic calculations   |
| **Hairpin (3' end)** |         -15        | Primer folds on itself at critical 3' end      |
| **Self-dimer**       |         -15        | Primer binds to itself                         |
| **Hetero-dimer**     |         -10        | Forward and reverse primers bind to each other |
| **Weak GC Clamp**    |         -15        | No G/C at 3' end (reduced specificity)         |
| **Strong GC Clamp**  |         -5         | Too many G/C at 3' end (may cause mispriming)  |
| **Poly-X Run**       |         -10        | Repetitive nucleotides (e.g., AAAA, GGGG)      |

***

## Understanding Warnings

Warnings do not necessarily mean the primer is unusable. Use this guide to interpret them:

### ⚠️ Common Warnings

| Warning               | Severity | Explanation                   | Recommendation                                    |
| --------------------- | :------: | ----------------------------- | ------------------------------------------------- |
| `Hairpin detected`    |  Medium  | Secondary structure may form  | Check if ΔG > -3 kcal/mol (usually OK)            |
| `Self-dimer detected` |  Medium  | Primer may dimerize           | Check if ΔG > -6 kcal/mol (usually OK)            |
| `Hetero-dimer risk`   |   High   | Primers may bind each other   | Reduce primer concentration, redesign if ΔG \< -8 |
| `Weak GC clamp`       |    Low   | 3' end lacks G/C              | Often acceptable for non-critical PCR             |
| `Poly-X run detected` |  Medium  | Repetitive sequence           | May cause polymerase slippage, consider masking   |
| `Off-target binding`  |   High   | BLAST found similar sequences | Verify with in-silico PCR, may need redesign      |

### ✅ When Warnings Are Acceptable

1. **Hairpin ΔG > -2 kcal/mol**: The structure won't form at PCR temperatures
2. **Self-dimer ΔG > -5 kcal/mol**: Weak interaction, unlikely to compete with template
3. **Weak GC clamp on one primer only**: The other primer can compensate
4. **Poly-X of 3 or less**: Generally not problematic

***

## Key Metrics Explained

### Melting Temperature (Tm)

| Tm Difference |     Status    | Notes                                   |
| :-----------: | :-----------: | --------------------------------------- |
|     ≤ 2°C     |    ✅ Ideal    | Both primers anneal at same temperature |
|     2-5°C     | ⚠️ Acceptable | May need gradient PCR to optimize       |
|     > 5°C     |   ❌ Problem   | One primer may not anneal efficiently   |

**Recommendation**: Keep Tm between 58-62°C for standard PCR, 60-65°C for long-range.

### GC Content

|        GC%       |     Status    | Notes                                |
| :--------------: | :-----------: | ------------------------------------ |
|      40-60%      |    ✅ Ideal    | Balanced stability                   |
| 30-40% or 60-70% | ⚠️ Acceptable | May need optimization                |
|  \< 30% or > 70% |   ❌ Problem   | Likely unstable or too tight binding |

### Delta G (ΔG) Values

ΔG measures the stability of secondary structures. **More negative = more stable = more problematic**.

| ΔG (kcal/mol) | Interpretation                               |
| :-----------: | -------------------------------------------- |
|      > -2     | ✅ No significant structure                   |
|    -2 to -5   | ⚠️ Minor structure, usually OK               |
|    -5 to -8   | ⚠️ Moderate structure, may affect PCR        |
|     \< -8     | ❌ Strong structure, likely to cause problems |

***

## QC Modes

PrimerLab supports three QC stringency levels:

| Mode         | Use Case                         | Penalty Severity              |
| ------------ | -------------------------------- | ----------------------------- |
| **strict**   | Diagnostics, clinical            | High penalties, low tolerance |
| **standard** | General research                 | Balanced                      |
| **relaxed**  | Exploratory, difficult templates | Lower penalties               |

Set in config:

```yaml theme={null}
qc:
  mode: standard  # or strict, relaxed
```

***

## When to Accept vs. Reject

### ✅ Accept the Primer Pair If

* Score ≥ 70
* No ❌ (critical) warnings
* Tm difference ≤ 3°C
* In-silico PCR produces single product

### ❌ Reject and Redesign If

* Score \< 50
* Multiple high-severity warnings
* Off-target amplification detected
* Tm difference > 5°C
* Product size outside expected range

***

## See Also

* [Scoring System](scoring) — Detailed algorithm explanation
* [Quality Control](/docs/guides/quality-control) — QC workflow guide
* [Troubleshooting](/docs/troubleshooting) — Common issues and fixes