A note on how Lateral-flow and colorimetric approaches gets discussed rather than on Lateral-flow and colorimetric approaches itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
On the subject in the title: Lateral-flow and colorimetric approaches: the specificity problem Working notes rather than a conclusion.
Lateral-flow and colorimetric approaches — I have the observation and I do not trust my interpretation of it, so I am posting the observation and holding the interpretation back.
Numbers, method and the conditions under which they were collected are below. Interpret them however they warrant.
The right question for any home test is: what physical quantity does it measure, and what would have to be true for that quantity to answer your question? Most disappointment comes from skipping it.
A note on how Lateral-flow and colorimetric approaches gets discussed rather than on Lateral-flow and colorimetric approaches itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.
Lateral-flow devices: like a rapid COVID test. They have a reagent strip and produce a colour result. They are quick but not precise and not intended for quantitative work.
Useful. I have added it to my own notes with the date on it.
Post #5 is the version of this I will quote in future. One addition.
The arithmetic on Lateral-flow and colorimetric approaches is the easy part and it is where the errors are, which is an uncomfortable combination. Show your working and someone will catch it.
Post #7 is right about the mechanism and I think understates the practical bit.
Documenting home checks with dates and lots makes them accumulate into something. Undocumented ones evaporate and get remembered selectively.
Happy to expand any of that if it is the useful part.
Offering a way to settle Lateral-flow and colorimetric approaches rather than another opinion about it. Two measurements, taken the same way, a fortnight apart. If the difference is within the noise, the question was not answerable at this precision.
Saving this. It is the version I will quote when the question comes round again.
A home refractometer is not measuring what people hope it is measuring in these preparations, and the readings are dominated by everything other than the peptide.
Someone should write this up properly, and it should probably not be me.
Building on post #12 rather than restating it.
Reading a colorimetric result at the wrong time is the commonest procedural error. Development is time-dependent and the window is usually narrow.
Post #12 describes the usual case. This is about the unusual one.
What physical quantity does the device measure? A colorimetric or lateral-flow method typically responds to a class of chemistry rather than to a specific molecule. It can distinguish "peptide present" from "nothing present" but generally cannot distinguish the intended peptide from a closely related one.
Colorimetric and lateral-flow methods generally respond to a class of chemistry rather than to a specific molecule. They can often distinguish something from nothing and rarely distinguish one peptide from a close relative.
A weak preference rather than a position.
Where I have landed on Lateral-flow and colorimetric approaches, having got it wrong once in public: the direction is clear, the magnitude is not, and anyone quoting a precise magnitude has borrowed it from somewhere that did not measure it.
The confident answers on Lateral-flow and colorimetric approaches and the well-sourced answers are not the same answers, which is the most useful thing I have learned reading this category.
Ultraviolet absorbance at 280 nanometres estimates concentration for peptides containing aromatic residues and gives nothing for peptides that do not. Knowing which yours is comes first.
Post #19 answers the question as asked. The question underneath it is different.
On Lateral-flow and colorimetric approaches: the maintained page in the documentation commons covers the general case with citations and a review date, which is more reliable than any reply here including this one.
Colorimetric tests: colour intensity correlates with concentration, but the relationship is non-linear at extremes and the correlation depends on the exact conditions (pH, temperature, timing). Reading the colour visually introduces subjectivity.
I had written a reply contradicting post #18 and deleted it. Here is what survived.
False positives and false negatives: no test has 100% sensitivity and 100% specificity. A negative result does not guarantee absence; a positive result does not guarantee presence. The predictive value depends on prevalence.
I would keep Lateral-flow and colorimetric approaches and the decision it usually gets used for separate in this thread. They are related and they are not the same question, and merging them is why the last one went badly.
Reading rather than answering, but this is the post I would point somebody at.
Where I part company with post #22, and it is a narrow parting.
The right question for any home test is: what physical quantity does it measure, and what would have to be true for that quantity to answer your question? Most disappointment comes from skipping it.
That is what the documentation says. What happens in practice is usually close.
Building on post #28 rather than restating it.
An update on my earlier Lateral-flow and colorimetric approaches post: the pattern held for another six weeks and then stopped, which I did not predict and cannot explain.
Distinguishing three things in the Lateral-flow and colorimetric approaches discussion that keep getting used interchangeably: the observation, the proposed mechanism, and the recommendation that gets attached to both.