Most FISH failures can be traced to one of five areas: specimen fixation/pretreatment, probe handling, denaturation, hybridisation, or post-hybridisation wash/imaging. Do not optimise several variables at once. Start with a known positive control, confirm the microscope/filter setup, then change one upstream variable at a time.
A failed fluorescence in situ hybridisation experiment rarely has only one possible cause. Weak signal, diffuse background, damaged nuclei and ambiguous signal patterns can all arise from different stages of the workflow. The fastest troubleshooting approach is to identify whether the problem affects all slides, one specimen type, one probe, or only one scoring region.
First question: did the positive control work?
If the known positive control fails, treat the run as a technical failure until proven otherwise. If the control works but one sample does not, prioritise specimen quality, fixation, tissue processing and sample-specific pretreatment rather than immediately blaming the probe.
Weak or absent FISH signal
Common causes
- Probe degradation, poor storage or repeated freeze–thaw exposure
- Insufficient target accessibility because pretreatment/protease digestion is too mild
- Inadequate DNA denaturation
- Hybridisation time or temperature not matched to the probe chemistry
- Excessively stringent post-hybridisation washing
- Wrong fluorescence filter set or poor microscope illumination
- Photobleaching from unnecessary light exposure
What to check first
- Confirm the probe storage history and expiry.
- Check the positive control under the correct filters.
- Verify the actual measured denaturation and wash temperatures with calibrated equipment.
- Review pretreatment/protease conditions against the specimen type and IFU.
- Confirm that the hybridisation duration matches the product — especially if a rapid workflow was substituted for an overnight method.
High background or diffuse fluorescence
Common causes
- Insufficient stringency washing
- Excess probe or inadequate removal of unbound probe
- Incomplete coverslip seal and evaporation during hybridisation
- Tissue autofluorescence
- Excessive exposure/gain during imaging
- Dirty slides, coverslips or immersion oil contamination
What to try
Confirm the wash buffer composition, temperature and timing before increasing stringency. If the run otherwise works, change one parameter at a time. Also review image acquisition: overexposure can turn modest background into an apparent assay problem.
Poor nuclear or tissue morphology
Common causes
- Over-digestion with protease/pepsin
- Excessive heat pretreatment
- Over-denaturation
- Poor fixation or prolonged/variable fixation
- Tissue sections detaching because slide adhesion is inadequate
For FFPE optimisation, morphology and probe penetration must be balanced. Increasing digestion can improve signal up to a point, then rapidly damage nuclei. Use adjacent sections and compare controlled pretreatment increments rather than making large changes.
Patchy hybridisation
- Air bubbles under the coverslip
- Probe pushed out by excessive coverslip pressure
- Uneven seal or evaporation
- Insufficient probe volume for the target area
- Uneven or folded tissue section
OGT/CytoCell specifically cautions that pushing too hard on the coverslip can squeeze probe out and cause patchy hybridisation. Remove visible bubbles before sealing.
Ambiguous break-apart or fusion pattern
Do not apply a generic “one or two signal diameters” rule across every product. Break-apart distance, fusion definitions, atypical patterns and cut-offs depend on the probe map, specimen and validated scoring method.
Check
- The exact probe map and fluorophore assignment
- The manufacturer's normal and abnormal signal examples
- Whether the nucleus is truncated or overlapping another nucleus
- Whether the signals lie in different z-planes
- Your validated definition of separated, fused or extra signals
- The established normal cut-off/background rate
Too many or too few signals
Unexpected copy number can be biological, but technical explanations should be excluded first. Section truncation can reduce apparent signals in FFPE; overlapping nuclei can increase apparent counts. Nuclear boundaries should be unambiguous and scoring should follow the product-specific assay rules.
Weak DAPI or poor focus
- Check counterstain concentration and mounting medium.
- Confirm the slide is mounted flat and free from bubbles.
- Use low-autofluorescence immersion oil for oil objectives.
- Confirm lamp/illumination alignment and filter condition.
- Avoid imaging through excessively thick or uneven tissue.
Troubleshooting matrix
| Observation | Most likely workflow area | High-value check |
|---|---|---|
| No signal on all slides | Probe handling / denaturation / microscope | Positive control, probe storage, actual denaturation temperature, filter set |
| Signal only weak on FFPE | Fixation / pretreatment | Heat pretreatment and protease optimisation |
| Strong signal + high haze | Wash / imaging | Stringency wash and exposure/gain |
| Patchy areas | Probe application / seal | Bubbles, probe volume, coverslip pressure, evaporation |
| Morphology destroyed | Pretreatment / denaturation | Reduce protease, heat or denaturation intensity |
| Borderline split/fusion | Scoring / sectioning | Probe-specific criteria, z-plane, truncation, controls |
A disciplined optimisation strategy
- Use a known positive and negative sample.
- Keep probe lot, microscope settings and wash conditions constant.
- Change one variable only.
- Record the exact measured temperature/time rather than just the instrument setting.
- Capture representative images for each condition.
- Choose the mildest pretreatment that gives strong, discrete signal while preserving morphology.
- Document the final condition as a specimen/probe-specific validated workflow.
Frequently asked questions
Why is my FISH signal weak after shortening hybridisation?
The probe chemistry may not support the shorter incubation. Return to the validated condition or use a probe/buffer system specifically designed for rapid hybridisation.
Can I increase probe concentration to fix weak signal?
Sometimes, but it can also increase background. First confirm specimen pretreatment, probe integrity, denaturation and hybridisation conditions.
Why is my FFPE tissue falling off the slide?
Harsh heat/protease conditions, inadequate slide adhesion or poor section handling can contribute. Use appropriate charged slides and optimise pretreatment intensity.
Should I score overlapping nuclei?
Generally avoid them unless your validated assay explicitly defines how. Overlap can create false copy-number and fusion patterns.
Explore more
For probe choice before optimisation, use our FISH probe selection guide. For a complete bench workflow, see the FISH protocol.
For method setup, start with the FISH protocol, then use the probe selection guide to confirm the signal design. If turnaround is the limiting factor, see FAST FISH vs conventional FISH.
ABMIUM lists 444 active Wuhan HealthCare Biotechnology FISH Probe Reagents in the catalogue. Browse FISH probes or send us your target and specimen type if you want help matching the probe and workflow.