HbA1c lag and what it can and cannot tell you at eight weeks — one year on

Posting this under the heading it deserves: HbA1c lag and what it can and cannot tell you at eight weeks — one year on Everything below is what sits behind that.

A methods question rather than a substantive one, about HbA1c lag.

Everyone quotes the same figure and I cannot find anyone who says how it was arrived at. That is not an accusation; it usually means the derivation is somewhere obvious and I have missed it.

A note on how HbA1c lag gets discussed rather than on HbA1c lag itself: the confident posts get the replies and the careful ones get ignored, and the careful ones have been right more often.

This is the sort of exchange that makes the archive worth searching.

I had written a reply contradicting post #2 and deleted it. Here is what survived.

TSH and free hormones: TSH is a reasonable screening test but does not tell you about actual free hormone levels. If TSH is abnormal, free thyroid hormones confirm whether there is a real thyroid problem.

Reference intervals: constructed to contain the central 95% of a reference population, which means one in twenty healthy people falls outside one by definition. Add biological variation and analytical imprecision and the base rate of a meaningless flag is substantial.

Post #2 describes the usual case. This is about the unusual one.

Lipid panel interpretation: total, LDL, HDL, triglycerides all on one panel. Reading them together is more informative than reading one value in isolation. An elevated triglyceride with low HDL is different from triglyceride elevation alone.

Post #4 is the version of this I will quote in future. One addition.

Counterpoint on HbA1c lag, offered without confidence: the same observation is consistent with a much duller explanation, and nobody has ruled the dull one out.

The arithmetic in post #7 is right; the assumption feeding it is the part to check.

A trend needs at least three points to be a trend. Two points are a line and a line through noise is still a line.

Answering the question post #6 raises rather than the one it answers.

The most useful thing this subcategory does is help people tell an interesting result from an uninteresting one before booking an appointment about it.

Adding it because I spent an afternoon working it out and nobody should have to twice.

Post #9 describes the usual case. This is about the unusual one.

Reference intervals are constructed to contain the central ninety-five per cent of a reference population, so one healthy person in twenty falls outside one by definition.

Seconded. It reads as careful rather than confident, which is the right register.

Time of day: some biomarkers vary across the day. Cortisol in the morning differs from cortisol in the evening. Comparing results from different times of day is comparing things that are not the same.

Two people can read the same figure differently here and both be reasonable.

Confirming post #13 from a second method, which matters more than confirming it from a second person.

Worth stating the null on HbA1c lag before we explain it: the observation may be nothing. That possibility deserves a sentence and usually does not get one.