The trend is what I care about; individual weeks are obviously noise.
I am trying to choose between two options that are usually discussed as though only one exists.
I am not optimising for price, but I am not indifferent to it either.
Which axes does this decision turn on?
8 weeks is 56 days, and the only question is whether 8 weeks accumulates more signal than the instrument's own noise. Scan-to-scan precision on lean mass is about one to one and a half per cent under standardised conditions, so on a 60 kg lean mass the least significant change is roughly 0.9 kg. At half a kilogram of total loss a week with a quarter of it lean — 0.125 kg of lean per week — 8 weeks accumulates 1 kg. 1 kg clears the 0.9 kg threshold, so a repeat every 8 weeks resolves a real change rather than reporting its own error. That is the interval to use. Standardised means the same machine, the same time of day, the same hydration state, the same clothing. Change any of those and the precision figure this arithmetic rests on is no longer yours. Three scans on a line beat two scans and a difference at any interval: the third is what tells you whether the second was a measurement or an outlier.
The relevant limitation is that lean mass includes water, so anything that shifts fluid shifts the reported lean figure without any tissue changing.
Standardise: same scanner, morning, twelve hours fasted, no exercise in the preceding twenty-four hours, no unusual carbohydrate intake in the preceding two days, and the same clothing.
On the detail: radiation exposure is very low — of the order of a few microsieverts, comparable to a day or two of natural background — so scan frequency is limited by cost and by precision, not by dose.
DXA precision error and least-significant-change calculations are standard quality metrics published by densitometry societies and by well-run facilities.
The caveat is that DXA is a two-compartment estimate in a three-compartment world, and its accuracy against reference methods is decent rather than excellent.
Same machine, same conditions, twelve weeks apart. Anything else is noise collection.
edited 10 Dec 2024 by sian_llewellyn — tightened the wording; no substantive change
Analytical standards and reagents with traceable certificates. Every quantitative result you read inherits the accuracy of the standard behind it.
Shop standardsRegional analysis — trunk versus limbs — is more informative than the total for anyone tracking training effects.
Typical precision error on a good DXA is around one per cent for fat mass and one to two per cent for lean mass, which means the least significant change is roughly two and a half to three per cent — about one to one and a half kilograms of lean mass for most people.
Worth being precise here: scanning more often than every twelve to sixteen weeks generally produces changes inside the precision error, which reads as noise and gets interpreted as signal.
Precision is good, accuracy is conditional. Track change, do not compare absolutes.
Answer first: dual-energy X-ray absorptiometry is precise and only conditionally accurate, so it is excellent for tracking change on the same machine and poor for comparing an absolute number against someone else's.
Bone mineral density falls modestly during sustained weight loss, and having the baseline on the same series is the only cheap way to see it.
On the detail: hydration is the dominant confounder. A litre of fluid is a kilogram, and it is allocated mostly to the lean compartment, so an under-hydrated scan flatters the body-fat percentage and a post-carbohydrate-loading scan does the opposite.
A body-fat percentage from one machine and one from another are not the same measurement and should never be compared.
Take the bone density seriously; it is the part of the scan nobody reads.
Start with standardising the conditions, because hydration, food, recent exercise and time of day all move the result by more than the change you are trying to detect.
Scans are usually reported with a total and with regional segments. Regional lean mass in the limbs is the appendicular skeletal muscle measure used in the sarcopenia definitions, and it is more useful than the total for training questions.
Effective dose for a whole-body composition scan is published at a few microsieverts, well below a chest radiograph.
Ask the facility for its precision error. If it does not have one, that is information.
The honest answer is that a single scan tells you very little and a series of three tells you a great deal.
Different manufacturers use different edge-detection and tissue-decomposition algorithms, so cross-machine comparisons can differ by several percentage points of body fat on the same person on the same day.
Nothing here is medical advice.
Read the regional segments, not just the total.
edited 10 Nov 2024 by nils_karlberg — updated for the 2026 guidance change
Ask PeptideStack is a static archive. Posting is closed, but the norms are worth stating: answer the question that was asked, show your working, cite the trial or the certificate, and say plainly where the evidence runs out.