Package a specimen for the laboratory
Dry it, breathe it, seal it, sign it. Learn why sealed plastic ruins biological evidence and paper does not.
- Biological material keeps changing after collection: A specimen is not frozen in time the moment it is bagged. Bacteria and fungi keep working on it, and what they leave behind is not what the laboratory was sent to find.
- Moisture is what growth needs: Warmth and damp together are growth conditions. Removing the damp is the cheapest control available at a scene.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now diagnose packages that arrived unusable, and protect a swab from the person taking it.
A swab was sealed damp into a zip bag and reached the laboratory three days later with visible fungal growth. Which handling step failed?
Reviewed- A.The swab was not refrigerated during transport
- B.The bag was not signed across the seal
- C.The swab was taken with a cotton tip instead of a synthetic one
- D.The swab was never dried before the bag was sealed
Show the worked solution ▾
Answer: D. The swab was never dried before the bag was sealed
- Step 1: Name what the growth needed: Fungal growth needs moisture, and three days at ambient temperature is plenty of time.
- Step 2: Find the step that supplied it: Sealing a damp swab in a non-breathable bag is what kept the moisture in contact with it.
Why it's right: Fungal growth needs moisture and time, and both were supplied by sealing a damp swab into a bag that could not let vapour out. Drying before packaging is the step that was skipped.
- A: Cooling would have slowed the growth, but the swab should not have been damp in a sealed bag in the first place. Refrigeration is the backup, not the fix.
- B: An unsigned seal is a documentation failure and a real one, but it does not grow fungus.
- C: Tip material affects how well cells release into a buffer. It does not decide whether a damp, sealed swab grows anything.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
While capping a cheek swab, the collector's bare fingers brush the cotton tip. Why does that matter for a DNA sample?
Reviewed- A.The oils on skin dissolve the cotton fibres
- B.The collector's own cells transfer onto the tip
- C.The tip becomes too warm to store
- D.The swab loses the moisture it needs for extraction
Show the worked solution ▾
Answer: B. The collector's own cells transfer onto the tip
- Step 1: Ask what fingers leave behind: Skin sheds cells continuously, and cells carry the collector's own DNA.
- Step 2: Ask where those cells now sit: On the exact surface the laboratory will extract from, mixed with the specimen it was collected for.
Why it's right: Skin sheds cells continuously, and those cells carry the collector's DNA. Once they are on the tip, the laboratory is extracting from a mixture and cannot fully separate the donor from the collector.
- A: Skin oils do not dissolve cotton. The transfer that matters is biological material moving onto the tip, not damage to the fibres.
- C: Brief finger contact does not meaningfully warm a swab, and temperature is a storage question rather than a contact one.
- D: A cheek swab is dried before packaging on purpose. Moisture is not something the tip has to retain.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student explains why a laboratory reported a mixed DNA profile on a swab that should have carried one person's material.
Fill these in as you work through the lesson.
- Breathable packaging (lets water vapour out):
- Degradation (the sample breaking down):
- Sealed and initialled (signed across the closure):
- Substrate (the material the stain is on):
Biological material is before packaging, then placed in packaging, then sealed and initialled across the closure.
- Give the reason a sealed plastic bag is the wrong container for a damp stain.
- Name what a signature across a seal tells the next person.
- Describe what you would do with an item too wet to dry at the scene.
A cloth carries a damp reddish stain. Air dry it out of direct sun and away from traffic. Fold it into clean paper, place that into a paper envelope, seal the envelope, and sign across the seal so the next person can see it has not been opened. The moisture leaves through the paper instead of sitting against the stain.