Internal Control Ct Delay

Screen sample internal-control Ct delay against the median reference and a supplied tolerance; delay is not specific for inhibition.

Description

Compare a sample’s internal-control Ct with the median reference Ct and flag delay strictly above a supplied cycle tolerance.

In quantitative PCR, a threshold cycle, commonly called Ct, records when amplification reaches a defined signal threshold. An internal control helps monitor aspects of the analytical process. Comparing its Ct with reference controls can reveal a delayed signal that deserves assay review.^[1]

A delay can arise from inhibition or other differences in input, extraction, or amplification conditions. This tool defines delay as sample-control Ct minus the median of supplied reference-control Cts, keeping that signed difference separate from the flag.

Internal Control Ct Delay returns the reference median, delay in cycles, and a boolean based on your maximum tolerance. The field name inhibitionFlag represents this screening comparison, not a demonstrated cause.

When to screen internal-control shifts

Use the tool with comparable sample and reference control measurements from a defined assay. Choose the tolerance outside the calculator and retain the assay’s control interpretation rules.

Summarize several reference Cts with a median.
Apply a consistent maximum-delay comparison to a sample.

How to enter control Cts and calculate delay

Enter positive Sample control ct and a nonempty Reference control cts list of positive finite values.
Enter nonnegative Maximum delay cycles.
Run the screen and read Reference median ct, Delay cycles, and Inhibition flag.
Review a flagged result with the underlying assay controls and procedure.

delayCycles = sampleControlCt − median(referenceControlCts); inhibitionFlag = delayCycles > maximumDelayCycles

The implementation sorts a copy of the reference list. For an even count it averages the middle two values. Equality with the maximum is not flagged.

json

{
  "input": {
    "sampleControlCt": 30,
    "referenceControlCts": [
      25,
      26,
      27
    ],
    "maximumDelayCycles": 2
  },
  "output": {
    "referenceMedianCt": 26,
    "delayCycles": 4,
    "inhibitionFlag": true
  }
}

A sample Ct below the median gives a negative delay and is not flagged by this rule. Empty references, nonpositive Cts, negative tolerance, and malformed or nonfinite inputs produce errors.

Limitations and interpretation

No diagnosis of inhibition, target concentration, control recovery, or assay validity follows from the comparison alone.
The tool uses supplied controls without run matching, outlier removal, amplification-efficiency correction, or uncertainty estimates.
An undetected control cannot be entered as a missing Ct or a made-up numeric ceiling without changing the analysis.
The tolerance is caller-defined; no assay-specific cutoff is built in.

Alternative or Complementary methods

Control shifts are one element of a broader assay review. A batch-screen record audit distinguishes missing tests and invalid-control flags, while a fitted detection-limit inversion answers a different analytical question.

References

The MIQE guidelines: minimum information for publication of quantitative real-time PCR experiments — Clinical Chemistry; S. A. Bustin and colleagues (2009)

Similar or alternative tools

RCL Batch Screen Audit
Audit required tests alongside supplied negative-result and valid-control flags.
Probit Detection Limit
Invert a separately fitted probit model for a target detection probability.