Two scans six months apart on one machine tell you something. Two scans six weeks apart on different machines tell you about the machines. Old habit: I write down the expected answer before I calculate it.
On the subject in the title: Two scans six weeks apart on different machines: a cautionary tale Working notes rather than a conclusion.
A question about measurement precision rather than about physiology.
Two readings, some weeks apart, on the same equipment, differing by an amount that feels meaningful and may not be. I do not know what this instrument's least significant change actually is.
How much difference does there have to be before it is a difference?
Picking up post #2: that is the part I would want checked first.
DXA (dual-energy X-ray absorptiometry): the standard for body composition in research. Precision is roughly ±1 kg for lean mass. Two scans six months apart on the same machine with the same technician give you useful information. Two scans six weeks apart on different machines tell you about the machines.
Posted with less confidence than the sentence structure implies.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
The short version is the first sentence; the rest is why.
Coming back to post #4, because the follow-up matters more than the original answer.
Photographs under fixed lighting and posture are a genuine record and are the hardest of these to standardise.
Two scans six months apart on one machine tell you something. Two scans six weeks apart on different machines tell you about the machines.
Old habit: I write down the expected answer before I calculate it.
Post #6 answers the question as asked. The question underneath it is different.
Bone mineral density changes with substantial weight loss are measurable and slow, and are one of the better reasons to have a baseline scan.
Quietly grateful for the plain phrasing. Not every thread gets that.
Coming back to post #7, because the follow-up matters more than the original answer.
DXA on the same machine with the same technician has a least significant change for lean mass of roughly a kilogram, sometimes more. Below that, two scans are the same scan.
Anyone who has looked at this more carefully, please correct the record.
Photographs under fixed lighting and posture are a genuine record and are the hardest of these to standardise.
I would be interested in a counterexample if anyone has one.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
The honest answer is that it depends, and here is what it depends on.
Bone mineral density changes with substantial weight loss are measurable and slow, and are one of the better reasons to have a baseline scan.
Take the reasoning and check the arithmetic; I do not always get it right.
Photographs under fixed lighting and posture are a genuine record and are the hardest of these to standardise.
If it helps: the failure mode here is usually boring rather than dramatic.
The most useful protocol is the one you will actually repeat identically. An imperfect measurement repeated consistently beats a better one repeated carelessly.
That is the version I use. It may not be the version that is correct.
Narrowing post #20, because the general version has more than one answer.
Neither method is capable of detecting the month-to-month changes that people report detecting month to month.
Post #22 and I disagree about the size of the effect, not about the direction.
Comparing a body composition result against a population reference is a different exercise from tracking your own change, and the second is what most people want.
That is one dataset and I would not build a rule on it.
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