Why direct PCR is not always working?

Quantitative PCR (qPCR) and reverse transcription qPCR (RT-qPCR) are fast techniques enabling detection and quantification of pathogens and nucleic acids. Direct amplification from crude samples avoids tedious extraction, but crude samples often contain natural PCR inhibitors that reduce efficiency, yield and reproducibility.

Short sample lysis combined with a swift protease digestion (for example using highQu SampleIN™ Lysis Set for PCR/qPCR) generates PCR/qPCR-ready templates. Crude extracts can be refrigerated and used for days to weeks.

Common PCR inhibitors in environmental & clinical crude samples

Inhibition minimization & direct PCR optimization (brief)

• Use robust, inhibitor-resistant PCR/qPCR mixes that contain inhibitor-reducing and PCR-enhancing agents.
• If direct PCR fails without lysis, perform dilution series (e.g. 1:10 and 1:100). Rule of thumb: less is better than more.
• Always run positive controls with different dilutions of crude extract to determine at which concentration inhibitors appear.
• If direct PCR fails, perform lysis per kit manual and repeat dilution series. Optimize lysis and PCR parameters.

General rapid lysis procedure (highQu SampleIN™ Lysis Set for PCR/qPCR)

1. Sample collection & guidelines

Note: Increasing sample amounts slightly can improve yield but too much material may cause inefficient lysis and inhibition. Plant materials often require thorough mechanical disruption; optimization recommended.

2. Sample DNA lysis protocol (example)

1. Use contamination-prevention measures: clean bench, gloves, sterile tubes.
2. Thaw DPK buffers at room temperature and mix well.
3. Prepare a 100 µl extraction reaction (use 3× volumes for buccal swabs):
   

   DPK Lysis Buffer, 5×	20 µl
   DPK Protease Buffer, 10×	10 µl
   PCR water (not supplied)	70 µl

4. Mix gently. Place in thermal block / water bath:
   Lysis: 75 °C — 5 min (vortex twice during lysis).
   Protease inactivation: 95 °C — 10 min.
5. Add 900 µl PCR water. Centrifuge 1 min to pellet debris. Transfer supernatant to a sterile tube and keep on ice.
6. Store at −20 °C for months or use immediately. Best results if PCR/qPCR performed immediately. Use supernatant directly as template (dilute 1:10 or more).

3. qPCR reaction setup (example)

Typical qPCR (20 µL total):

Controls setup

• No Template Control (NTC): detects contamination — replace template with water.
• No Reverse Transcriptase Control (−RT): for RNA templates — detects genomic DNA.
• Positive control: known pure template (purified or synthetic) to validate efficiency.
• Dilution control: test 1:10 and 1:100 template dilutions in duplicates/triplicates to reveal inhibitory effects.
• Internal Amplification Control (IAC): add a non-competitive synthetic target to detect inhibition and determine the concentration where inhibition occurs.

Template dilution advice

Start testing dilutions: 1:1, 1:10 and 1:100. Choose the lowest dilution that still gives robust amplification with acceptable Cq values. In RT-qPCR, avoid over-dilution that reduces sensitivity.

How to maintain PCR yield & reproducibility

• Keep the same optimized reaction setup and reagents (even same water) once established.
• Run reactions in duplicates or triplicates to reduce pipetting variance.
• Mix reagents well before use and maintain consistent sample input across replicates.
• Standardize lysis & dilution procedures. Avoid repeated freeze-thaw cycles — store crude extracts at +4 °C for short-term use.

Direct PCR/qPCR troubleshooting (quick guide)