Lab Insights / DNA and RNA extraction / Quality control
Is Your DNA Good Enough? NanoDrop, Fluorometry and DNA Integrity Explained
A DNA sample can have an impressive concentration and apparently normal purity ratios yet still fail PCR or sequencing. Here is how to distinguish DNA quantity, chemical purity, molecular integrity and functional suitability.
The core principle: No single DNA quality-control measurement answers every question. Use UV absorbance to screen purity, a DNA-specific fluorometric assay to estimate relevant DNA mass, fragment analysis to assess integrity, and an application-appropriate functional test where needed.
1. Define what “good DNA” means for your assay
Before repeating an extraction, ask what the DNA must do. A short-amplicon PCR assay may succeed with fragmented material that would be unsuitable for a high-molecular-weight long-read sequencing workflow. A library preparation may require accurate input mass and a particular fragment distribution. Sample acceptance criteria should therefore come from the downstream method, not from a universal purity cut-off.
It helps to separate five properties: concentration (DNA per unit volume), yield (total recovered DNA), purity (the presence of co-extracted materials), integrity (fragment size and degradation), and amplifiability or functional suitability (how the material performs in a relevant assay). A sample may perform well on one measure and poorly on another.
2. What does a NanoDrop measurement mean?
Microvolume UV-Vis spectrophotometry estimates nucleic-acid concentration from absorbance near 260 nm. It is quick, consumes very little sample, and provides a useful spectrum and ratios such as A260/A280 and A260/A230. However, UV absorption is not specific to intact double-stranded DNA. Other nucleic acids and some contaminants can contribute to the reading.
For relatively clean double-stranded DNA, A260/A280 near 1.8 and A260/A230 near 2.0 to 2.2 are frequently used reference indicators, not absolute acceptance criteria. A low A260/A230 may suggest extraction-reagent carryover or matrix-derived compounds; a low A260/A280 may raise questions about protein, phenol or other interference. Spectrum shape, sample concentration, the blank solution and instrument limitations all matter. Ratios become especially unstable for low-absorbance samples.
Bench check: Use the appropriate matched blank, inspect the complete spectrum rather than the ratio alone, confirm pedestal cleanliness and repeat an anomalous reading. Do not interpret a single low-concentration absorbance ratio as proof of contamination.
3. Why NanoDrop and fluorometry disagree
A DNA-selective fluorescent dye is designed to respond preferentially to a target nucleic-acid class, such as double-stranded DNA. When properly calibrated and used within the assay range, dsDNA fluorometry is generally more informative for the mass of dsDNA going into a PCR or library preparation. It does not, however, tell you whether the DNA is intact or free of every inhibitor.
If the UV-Vis result is substantially higher than the dsDNA fluorometric result, consider RNA carryover, free nucleotides, chemical interference or other non-dsDNA contributions. Also check assay range, dilution arithmetic and measurement technique. A discrepancy is a diagnostic clue, not a direct quantitative test for a specific contaminant.
| Question | Preferred measurement | What it cannot establish |
|---|---|---|
| How much nucleic-acid-associated absorbance is present? | UV-Vis spectrum and A260 estimate | Specific dsDNA mass or intact DNA |
| How much dsDNA is available? | Appropriate dsDNA fluorometric assay | Integrity or inhibitor absence |
| How fragmented is the sample? | Gel or capillary electrophoresis, selected for fragment range | Complete chemical purity or amplifiability |
| Can it support my downstream test? | Application-specific control or QC assay | Suitability for every other application |
4. Check integrity when fragment length matters
Fluorometry can quantify a severely fragmented sample. To evaluate integrity, use an appropriately selected agarose gel, capillary electrophoresis system or other fragment-sizing method. Inspect whether DNA is concentrated at a high molecular weight or distributed as a smear, and whether the method can resolve the size range relevant to the assay.
Conventional agarose gels are useful for many routine DNA preparations, while specialised high-molecular-weight methods may be necessary for long-read applications. For some short-fragment or cell-free DNA protocols, the important question is the proportion of DNA within a defined size window, not whether a sample appears as a single high-molecular-weight band. DNA integrity number or fragment-size thresholds are protocol-dependent, not interchangeable across applications.
For a refresher on electrophoretic separation and interpretation, see Agarose Gel Electrophoresis, Part 1 and Part 2.
5. Troubleshooting: the result looks good but PCR fails
| Observation | Possible explanation | Discriminating next step |
|---|---|---|
| High UV concentration, low fluorometric dsDNA | Non-dsDNA absorbance, contamination, or measurement-range error | Check blank and full spectrum; repeat fluorometry in range |
| Normal purity ratios, weak PCR signal | Inhibitors, low target copy number, fragmented target, or assay failure | Review controls, amplify a suitable control target and assess dilution response |
| Acceptable concentration, extensive smear | Fragmentation, nuclease activity or poor preanalytical handling | Inspect collection and storage history; assess fragment sizes relevant to intended assay |
| Variable yields across similar samples | Heterogeneous input, inconsistent lysis, recovery or measurement | Normalise input, inspect sample matrix and track extraction/QC controls |
| Unexpectedly low A260/A230 | Matrix compounds or reagent carryover; possible blanking artefact | Check full spectrum, blank and purification process before selecting cleanup |
An informative inhibition check can compare an undiluted extract with a diluted extract, but interpretation must account for the fact that dilution lowers both template and inhibitor concentrations. Internal amplification controls, extraction negatives and appropriate positive controls are important. A later Lab Insights guide will examine PCR inhibition in more detail. For reaction-level troubleshooting, read Fine-Tuning Your PCR Reactions.
6. A practical DNA QC sequence
- Identify the downstream method. Check its required DNA type, input amount, fragment-size distribution and any specified QC assays.
- Confirm sample history. Record specimen type, collection and storage conditions, input mass or volume and extraction method.
- Measure appropriate DNA quantity. Use dsDNA-specific fluorometry where relevant to the method and keep dilution factors traceable.
- Screen for chemical interference. Inspect UV absorbance and spectrum quality where sample amount permits; do not rely on ratios in isolation.
- Assess integrity if necessary. Choose a sizing technique capable of detecting relevant degradation or fragment distributions.
- Apply functional acceptance criteria. Use downstream controls or assay-specific QC, record deviations and decide whether re-purification, re-extraction or a different application is justified.
Illustrative workflow, not a substitute for a validated laboratory procedure or the acceptance requirements specified by the downstream assay manufacturer.
7. Frequently asked questions
Does an A260/A280 ratio of 1.8 prove that DNA is pure?
No. It is a useful screening indicator, but some contaminants are not reliably identified by that ratio. Inspect the spectrum and consider the sample matrix and intended assay.
Which result should I use for DNA input: NanoDrop or fluorometry?
Follow the downstream protocol. Many dsDNA-based workflows prefer a dsDNA-selective fluorometric result for mass calculations, but some sample types and assays require size-aware measurements or additional QC.
Can fragmented DNA still be used for PCR?
Sometimes. Success depends on target length, damage, template abundance, inhibitors and the assay. Fragmentation is more problematic when the required intact target or molecule is long.
Does a high DNA yield mean the extraction worked well?
Not necessarily. High recovered mass can coexist with inhibitors, substantial fragmentation or poor reproducibility. Interpret yield alongside QC and functional performance.
Should I re-extract when NanoDrop and fluorometry disagree?
Not automatically. First rule out measurement artefacts and determine which result is relevant to the next process. Repeat extraction only when evidence suggests the existing extract is unsuitable and the original sample permits it.
Connecting QC results to extraction-kit selection
When extracted DNA repeatedly fails a downstream QC requirement, investigate sample quality and the purification method together. Browse the Gene Vantage genomic DNA extraction range or request a kit recommendation based on your specimen, throughput and application. Review the product’s stated sample type and grade before use; changing kits is not guaranteed to resolve preanalytical damage.
References and further reading
- Thermo Fisher Scientific. DNA and RNA quantification: UV-Vis and fluorescence measurement principles.
- Thermo Fisher Scientific. NanoDrop nucleic-acid purity ratios and spectra.
- Oxford Nanopore Technologies. Input DNA/RNA QC guidance, revision October 2025.
- Illumina. DNA input recommendations for 5-Base DNA Prep.
- Merck. Genomic DNA purification and assessment.
Technical information reviewed against manufacturer guidance available in November 2026. References describe general principles; verify the current instructions for your specific downstream assay.