See Discussion section.

https://www.beckmancoulter.com/download/file/phxA18555AR-DE_...
https://www.beckmancoulter.com/wsrportal/techdocs?docname=/c...

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Note added at 1 day 6 hrs (2019-08-23 02:56:30 GMT)
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If you want to follow the Beckman–Coulter nomenclature, then it would be "Triglyceride-GPO (Leerwert)".
Perhaps that's a good option for laboratory staff or other experts working in that field. For anyone else I think it would be rather hard to interpret.

By my reading of the Beckman–Coulter method statements, what this is actually intended to mean is the level/concentration of triglycerides, according to the following technique:
(i) Through use of a "triglycerides GPO reagent" that will convert triglycerides to glycerol, determine the apparent content of triglycerides in a synthetic sample [the "blank"] that is known to contain no triglycerides, but instead contains glycerol at an appropriate concentration.
(ii) Through use of a "triglycerides GPO reagent" that will convert triglycerides to glycerol, determine the apparent content of triglycerides in a real (biological) sample that is expected to contain both triglycerides and glycerol.
(iii) Estimate the true content of triglycerides in a real (biological) sample by subtracting the result of analysis (i) from the result of analysis (ii).

I'm not entirely sure, but it seems that the "triglycerides GPO reagent" should contain a few things:
"Triglycerides in the sample are hydrolyzed to glycerol and free fatty acids by the action of lipase. A sequence of three coupled enzymatic steps using glycerol kinase (GK), glycerophosphate oxidase (GPO), and horseradish peroxidase
(HPO) causes the oxidative coupling [...] to form a red [...] dye."

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Note added at 1 day 6 hrs (2019-08-23 03:00:31 GMT)
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...So I don't think the reported value is the concentration of GPO present (I think GPO is a component of the "reagent"), nor do I think the blank/control value is being reported (I think the blank value has been applied to obtain the reported value).

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Note added at 1 day 6 hrs (2019-08-23 03:08:05 GMT)
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Amusingly, I came up with "Leerwertkorrektur" before discovering that it is a term often used by Beckman–Coulter!
https://www.linguee.com/english-german/search?source=auto&qu...

I wouldn't be surprised if there are some other more idiomatic options for this portion of the translation.
Borrowing an idea from Barbara Schmidt, "Blindwertkorrektur" is used sometimes
https://www.linguee.com/english-german/search?source=auto&qu...
Still not very often, yet significantly more often than "Leerwertkorrektur".

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Note added at 1 day 6 hrs (2019-08-23 03:09:52 GMT)
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"[...] intended for quantitative determination of triglycerides concentration in human serum or plasma."
"[...] dient [...] der quantitativen Bestimmung der Konzentration an Triglyceriden in Humanserum oder -plasma."

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Note added at 1 day 8 hrs (2019-08-23 04:56:43 GMT)
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I acknowledge Cornelia Litzmann's helpful post on the TranslatorsCafé forum.