Prevent cross contamination between samples
One sample at a time, one applicator per sample, and nothing goes back in the stock bottle. Learn why a carried-over trace reads as a real result.
- A trace is enough to give a result: Presumptive tests are built to detect very small amounts, so a carried-over trace is not too small to matter. It is exactly the size the test was designed to find.
- A contaminated result looks like a real one: Nothing on the plate or in the tube announces that the material came from the previous sample.
Prerequisites are inferred: pending teacher review.
Re-learn the skill with worked practice and clear examples.
Now read a result that may have come from the sample before it, and say what it can and cannot support.
Sample B tests positive. The same wooden applicator had been used on sample A moments earlier. What does the B result establish?
Reviewed- A.That A and B came from a common origin
- B.That B is positive, since the applicator was only briefly in A
- C.That B is negative, since the reaction came from A
- D.Nothing yet, since carryover could explain it
Show the worked solution ▾
Answer: D. Nothing yet, since carryover could explain it
- Step 1: List what touched both: The applicator went into A and then into B, so it is a route between them.
- Step 2: Ask whether the two explanations can be told apart: A true positive in B and carryover from A produce the same visible reaction, so this run cannot choose between them.
Why it's right: Carryover and a genuine positive produce an identical reaction, so this run cannot separate them. The honest conclusion is that the B result is unusable, and the test is repeated with a fresh applicator.
- A: A shared origin is a conclusion about the samples. Here the shared thing is a piece of equipment, which says nothing about where the samples came from.
- B: Brief contact is still contact, and presumptive tests are built to detect very small amounts. Duration does not rescue the result.
- C: Calling it negative is the same overclaim in the other direction. An unusable result does not become a negative one.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
A procedure calls for a glove change between two specimens even when neither glove is damaged. What is the reason?
Reviewed- A.Glove material weakens after a few minutes of wear
- B.A second pair gives a better grip on wet containers
- C.The exterior already holds traces of the first specimen
- D.The procedure requires an even number of gloves per run
Show the worked solution ▾
Answer: C. The exterior already holds traces of the first specimen
- Step 1: Treat the glove as a tool: A glove that handled the first specimen carries part of it, exactly as a used applicator does.
- Step 2: Ask where that tool goes next: Straight to the second specimen, which is the definition of a transfer route.
Why it's right: A glove that handled the first specimen carries traces on its exterior and then goes straight to the second, which makes it a transfer route just like a reused applicator. Damage is a separate reason to change, not the only one.
- A: Examination gloves do not lose their barrier within a normal task, and material fatigue is not what the change interval is set by.
- B: Grip is a comfort matter and does not change between a used pair and a fresh one of the same type.
- D: There is no counting rule. The change point is set by the move from one specimen to the next.
Aligned to Handling, Preparation, Storage and Disposal · reading level ~grade 9
- A student explains an unexpected positive on a scene sample by walking back through every object that met more than one container.
Fill these in as you work through the lesson.
- Cross contamination (one sample arriving in another):
- Carryover (what a reused tool brings with it):
- Stock bottle (the shared source of a reagent):
- False positive (a result that says yes when the truth is no):
One sample at a time, a fresh for each sample, and reagent poured out never goes into the stock bottle.
- Name three objects that can carry material from one sample into the next.
- Explain why a false positive from carryover cannot be spotted by looking at the result.
- Write what you would do with 2 mL of reagent you poured out and did not use.
You test sample A with a wooden applicator and get a colour change. You pick up the same applicator for sample B. Even after a wipe, the tip holds enough of A to produce the same colour change in B. The B result now says nothing about B, and there is no way to tell that from looking at it.