Internal Control Failure on a Stool Pathogen Panel
A 58-year-old man returns from two weeks in South Asia with six days of watery diarrhea (8-10 stools/day) and low-grade fever. The laboratory runs a multiplex real-time PCR stool panel (Salmonella, Campylobacter, Shigella/EIEC, STEC, norovirus GI/GII, rotavirus, Cryptosporidium, Giardia) with an armored-RNA internal control (IC) added at the lysis step. Extraction uses 200 microliters of unpreserved stool eluted into 100 microliters. On this specimen the IC amplifies at Ct 34.1; the validated acceptance criterion is IC Ct of 30.0 or lower. All twelve pathogen targets are negative. Re-testing the same eluate gives IC Ct 33.6, again with all targets negative. The rest of the 15-sample extraction batch showed IC Ct values of 25.9-27.4. The specimen was received 30 hours after collection and is described as formed rather than liquid.
1. Is the negative result reportable? What are the plausible causes of the elevated IC Ct, and what is the first thing you check to narrow them?
Read the worked answer
Not reportable. An IC outside the acceptance limit invalidates the run for that specimen; a negative result under these conditions cannot be interpreted as "no pathogen." Report the specimen as inhibitory/indeterminate with a comment and request a repeat.
First check: the batch pattern. The other 15 extracts had IC Ct 25.9-27.4, so the master mix, primer/probe, and IC lot performed normally. The problem is specimen-specific, not run-wide.
Specimen-specific causes:
- PCR-level inhibition by co-purified heme, bile salts, complex polysaccharides, or urea.
- Lysis/extraction failure - formed stool with tough matrices (gram-positive-rich flora, fibrous material) can leave nucleic acid unavailable even though the IC was added.
- Inhibitor carryover from the extraction chemistry (guanidinium/ethanol residual in the eluate).
- Pre-analytical degradation - 30 hours at ambient temperature is outside the validated window for some targets, though armored RNA should not itself degrade.
- Simple pipetting error of IC into that one well - check the extraction worksheet and whether the IC was added before or after the sample.
2. Design a spike-in experiment that distinguishes PCR-level inhibition from an extraction/lysis problem, and state how you would interpret each outcome.
Read the worked answer
Use the same IC lot at a defined copy number and place spikes at two points:
- Post-extraction spike: add IC directly to 10 microliters of the patient eluate and amplify without extraction. Run in parallel a clean-stool eluate spiked identically.
- Pre-extraction spike: add the same IC amount to a fresh aliquot of the original stool and carry it through lysis and extraction.
Interpretation:
- Post-extraction spike inhibited (Ct more than ~1.5 cycles later than the clean-stool control) but pre-extraction spike normal: inhibitor is present in the eluate and acts at the amplification step. Dilution or a clean-up step will help.
- Post-extraction spike normal but pre-extraction spike inhibited: the inhibitory material is generated or co-purified during extraction, or lysis is incomplete. Diluting the eluate will not fix the root cause; re-extract with a different chemistry (additional wash, bead-beating lysis, inhibitor-removal column, or larger elution volume).
- Both normal: the original failure was procedural (IC not added, short pipetting, thermal error) - re-extract and retest.
A dilution series of the eluate (1:5, 1:10) is a useful adjunct: if IC Ct drops by more than the 2.3 and 3.3 cycles predicted by simple dilution, inhibition is being relieved.
3. A 1:10 dilution of the eluate restores the IC to Ct 27.0 but all targets remain negative. How do you report, and what have you given up? What would you do differently given the travel history?
Read the worked answer
The dilution establishes that the specimen is amplifiable, so a negative at 1:10 is more credible than the original - but it is still not equivalent to a validated neat result.
What is lost: a 1:10 dilution shifts every target by roughly 3.3 cycles, so the effective limit of detection rises about one log (e.g., a validated 1,000 CFU/g becomes ~10,000 CFU/g). Norovirus and Shigella can be present at low titer, so a negative at 1:10 does not exclude them.
Report: "Negative at 1:10 dilution following inhibition of the neat extract; sensitivity reduced approximately one log. Repeat specimen recommended." Do not silently convert this into an unqualified negative.
Given the travel history, do not rely on a single inhibited molecular result. Request a fresh liquid stool and run both the molecular panel and culture (Salmonella, Shigella, Campylobacter) plus a Giardia/Cryptosporidium antigen or microscopy, since culture and antigen are unaffected by PCR inhibitors. Consider the optimal approach to be re-extraction with an inhibitor-removal method and mechanical lysis rather than dilution alone.