SPUD Assay for Detection of Assay Inhibitors Protocol

Quantitative PCR Protocols

Although quantitative PCR uses the same basic concept as traditional PCR, the reactions differ in that the amplicons are generally smaller and are detected indirectly using an additional dye or labeled probe or primer.

The SPUD assay is one option for identification of inhibitors that may be present in RNA or DNA samples. The assay is particularly useful when a large number of samples are to be analyzed or when targets are present at low copy number making dilution of the sample impractical. The test is used to avoid false negatives and to lend greater confidence to reporting of data based on a negative result.

The SPUD assay consists of an artificial template and two primers that are specific to the template and is run in the presence of SYBR® Green I dye or a template specific probe. During the assay, the SPUD template is amplified, resulting in a characteristic Cq. Alongside this control reaction, the artificial template is spiked into samples and measured in comparison to the control. In the presence of a clean sample, the Cq will remain the same as the SPUD control whereas in the presence of a contaminated sample, the Cq will shift to higher cycles.


  • Quantitative PCR instrument
  • Microcentrifuge
  • Laminar flow hood for PCR set up (optional)



  • cDNA/gDNA (or other sample for inhibitor test) diluted as for use in qPCR (usually 1:10). For the purposes of the following example protocol, cDNA is referred to as the test sample. However, any material may be monitored for inhibitory compounds. In this example, the amplification is monitored using SYBR Green I detection but a probe may be used as an alternative (listed as optional probe) and the reagents should be amended to a suitable probe detection master mix (e.g., LuminoCt qPCR ReadyMix, Sigma L6669).
  • KiCqStart SYBR Green I ReadyMix (Sigma KCQS00/KCQS01/KCQS02/KCQS03—see Table P4-6A to select appropriate reagents for the instrument) or LuminoCt qPCR ReadyMix (Sigma L6669) if a probe is used for detection.
  • PCR grade water: PCR grade water (W1754 or W4502) as 20 mL aliquots; freeze; use a fresh aliquot for each reaction.
  • Forward and Reverse primers for SPUD assay (stock at 10 μM; see Table P8-20). Optional SPUD probe FAMTGCACAAGCTATGGAACACCACGT-BHQ1 for use with a probe compatible LuminoCt® ReadyMix™ (Sigma L6669).
  • SPUD template oligo diluted to approximately 20,000 copies/μL from a stock concentration of 100 μM: Dilute 100 μM through 1:10 dilution series and test the 6th to 10th dilution for the concentration that yields a Cq of approximately 25. Dilute template from a 1:1,000× stock for each test.

Table P8-20. SPUD Assay Oligo Sequences.

Primer Sequence (5’-3’)



1.    Place all reaction components on ice.

2.    Mix and then centrifuge briefly to collect contents at the bottom of the tube.

3.    Determine the total number of samples to be tested in duplicate and also allow for two No Test Sample Controls.

4.    Prepare master mix for all samples and controls according to Table P8-21 allowing 10% extra to allow for pipetting
       error. Mix well.

Table P8-21. qPCR Master Mix for SPUD Inhibitor Test.

Reagent Volume (μL) per
Single 25 μL Reaction
2× KiCqStart ReadyMix 12.5
SPUD Primer F (10 μM stock 0.25
SPUD Primer R (10 μM stock) 0.25
SPUD template (pre diluted to 20,000 copies/μL) 1
Reference dye (instrument specific, see Table P4-7) 1
PCR grade water 5


5.    Aliquot 20 μL of SPUD qPCR master mix into the sample tubes/wells. If using a PCR plate, follow a plate schematic to
       ensure that the reagents, samples and controls are added to the right wells.

6.    Aliquot 5 μL cDNA sample into qPCR tubes/wells and 5 μL water to the No Test Sample Control tubes/wells.

7.    Cap tubes or seal the PCR plate and label. (Make sure the labeling does not obscure instrument excitation/detection
       light path.)

8.    Run samples using the two-step protocol provided below, repeating steps 1–2 for 40 cycles.

Table P8-22. qPCR Cycling Conditions for SPUD Inhibitor Test.

Cycling Conditions Temp (°C) Time (sec)
Initial Hot Start/denaturation 95 30
Steps 1–2 are repeated through 40 cycles
Step 1 95 5
Step 2

Note: Use standards dissociation curve protocol (data collection)