Meaning of "lower limit of normal + 20% over basal value" in a description of a blood lactate analysis (after glucose load)

Question

From "Alpers- and MNGIE-like disease with disturbed CSF folate transport and an unusual mode of genetic transmission of POLG mutations: a case report":

Basal serum lactate was normal (1.1–1.6 mmol/l); two hours after an oral glucose load, an abnormal increase to 3.11 mmol/l (lower limit of normal +20% over basal value) was noted. Also, serum lactate was found to increase (3.0 mmol/l) repeatedly during night-time vomiting.

I googled up the normal range for lactate, and it's 0.5-1.0 nmol/l. So, "lower limit of normal + 20%" is 0.5 + 0.1 = 0.6? Adding this to the upper bound of the mentioned basal value, 0.6 + 1.6, only makes up to 2.2.

I generally can't understand the meaning of the phrase. It seems quaint and there is seeminlgy no reason to construct such an explanation in parentheses. Is there any tradition or rule for estimation of after-load lactate, and how does the mentioned phrase fits in there?

I would expect to see "upper limit of normal + something" to justify calling of a lactate level "elevated".

Answer 1

I received an answer from Rudolf Korinthenberg (University of Freiburg), one of the authors:

The normal value for serum lactate is 0.5 – 2.2 mmol/l. We have measured this repeatedly and found all the single values between 1.1 and 1.6 mmol/l, thus they all were in the normal range.

To find an abnormality that is not visible in the everyday serum lactate values, one can make an oral glucose loading test (the same as is done on the suspiciion of diabetes mellitus). Here not only serum glucose, but also lactate will rise a little bit. The normal limits for this rise of lactate are not absolute, but they depend on the basal lactate level before application oft he glucose load. Normally, lactate should not rise higher than 20% of the basal level; so in our patient, if the basal lactate was 1.6 mmol/l, the highest level after the glucose load still tob e considered normal would have been 1.92 mmol/l. Thus his rise to 3.2 was clearly abnormal.

Hence, the meaning is (lower limit of normal: basal value + 20%).