Pleural Fluid LDH Unit Converter

Convert pleural fluid LDH between U/L, IU/L, µkat/L, nkat/L and µmol/s/L — and see why Light’s criteria specify two-thirds of your own laboratory’s upper limit rather than an absolute number.

Last updated 8 Oct 2026

Pleural Fluid LDH converter

Activity ⇄ katal

Pleural fluid LDH

An enzyme activity, not a mass — so there is no molar conversion, and the U/L to µkat/L factor is a definition rather than a molecular weight.

5.333µkat/LExample

Pleural fluid LDH 320 U/L, with a serum LDH of 210 U/L and a local upper limit of 225 U/L

The conversion is a definition, and the threshold is a fraction

1 U = 1 µmol/min · 1 kat = 1 mol/s
µkat/L = U/L ÷ 60 = U/L × 0.0166667
nkat/L = U/L × 16.6667 · µmol/s/L is the same quantity as µkat/L
Light’s absolute LDH criterion: fluid LDH > ⅔ × the laboratory’s own serum LDH upper limit of normal

÷ 60: the whole conversion. One enzyme unit is one micromole per minute and one katal is one mole per second, so a per-minute rate becomes a per-second rate by dividing by sixty. No molecular weight enters anywhere
U/L = IU/L: identical. The ‘international’ refers to the agreed definition of the enzyme unit, not to a separate potency standard, so the two spellings are one quantity — and µmol/s/L is likewise the same quantity as µkat/L
two-thirds of YOUR ULN: not two-thirds of the patient’s serum LDH, and not an absolute figure. LDH activity is method- and temperature-dependent, so a fraction of the local upper limit is the only form of the criterion that travels

Worked example

Pleural fluid LDH 320 U/L, with a serum LDH of 210 U/L and a local upper limit of 225 U/L
320 ÷ 60 = 5.333 µkat/L
320 U/L = 320 IU/L = 5,333 nkat/L = 5.333 µmol/s/L
LDH ratio = 320 ÷ 210 = 1.52, above 0.6 — exudate
Absolute criterion: ⅔ × 225 = 150 U/L, and 320 exceeds it — exudate again. Against Awanui's 250 U/L upper limit the threshold would be 167 U/L; against the pre-2020 Exeter method's it would have been nearer 270

Why the criterion is a fraction: what laboratories publish for serum LDH

Four published adult upper limits spanning 214 to 250 U/L give Light thresholds from 143 to 167 U/L, so a pleural fluid LDH of 155 U/L is an exudate at one of these laboratories and a transudate at another. Two of the largest reference laboratories decline to publish a number at all — not evasion, but the correct response to a measurand whose value depends on the method, and precisely why Light specified a fraction of the local limit. Reference intervals are method- and laboratory-dependent, and the interval printed on your own report takes precedence over any figure here.
Laboratory	Adult serum LDH interval	⅔ of the upper limit
Exeter Clinical Laboratory, males 16 and over	135 – 225 U/L	150 U/L
Exeter Clinical Laboratory, females 16 and over	135 – 214 U/L	143 U/L
Awanui Labs, 10 years and over	120 – 250 IU/L	167 U/L
Awanui Labs, previous all-ages interval	110 – 220 IU/L	147 U/L
Mayo LDBF, body fluid	no numeric interval published — an interpretive report	—
ARUP, serum LDH	no numeric interval published — by report, varies with instrumentation	—

How far a method change can move the same sample

The first row is the strongest argument on this page. One laboratory’s LDH results fell about 45% overnight on a method change, with no change in any patient: an absolute cut-off of 200 U/L would have inverted its meaning, while two-thirds of that laboratory’s own upper limit moved with the method and kept it. The last row is the contrast — a unit conversion changes the number and not the quantity, which is what makes it safe.
Change	Effect on reported LDH	Effect on the Light threshold
Exeter, method change of 15 December 2020	about 45% lower than the previous method	falls with it — the fraction tracks the method
Awanui, reference-interval harmonisation of February 2023	unchanged; the interval was re-partitioned by age	adult threshold moved from 147 to 167 U/L
Any absolute threshold memorised in U/L	—	wrong after either change
U/L to µkat/L	no change in the quantity	none — it is a definition

An activity, not a concentration, and what follows from that

Lactate dehydrogenase is not reported as an amount. What the laboratory measures is a rate — how fast the enzyme in the sample turns pyruvate and NADH over under defined conditions — and the result is an activity. One enzyme unit is one micromole of substrate per minute; the SI unit is the katal, one mole per second. So U/L to µkat/L is a division by sixty and nothing else. The molecular weight of LDH, about 144 kDa, does not enter, and there is no molar concentration to convert to. A reader who has looked for an LDH factor involving a molar mass has been looking for something that does not exist.

The consequence runs deeper than the arithmetic. An activity depends on the conditions of the assay — the buffer, the substrate concentration, the direction the reaction is run in, and the temperature — so change the method and the same sample gives a different number. This is not theoretical: one laboratory’s published note records that its current LDH method gives results about 45% lower than the one it replaced, so a cut-off of 200 U/L would have meant opposite things on either side of a single December afternoon. It is also why neither Mayo nor ARUP publishes a numeric reference interval for LDH at all — Mayo returns an interpretive report for body fluid LDH, and ARUP’s serum LDH is listed as ‘by report’, varying with instrumentation.

Light’s criteria were built to be immune to this. Two of the three are ratios — fluid LDH above 0.6 of serum LDH, fluid protein above 0.5 of serum protein — and the third is a fraction of the laboratory’s own upper limit of normal rather than an absolute figure. The ⅔ carries no units, so it survives a method change, an instrument change and a move between continents. The published adult upper limits collected here span 214 to 250 U/L, putting the threshold anywhere between 143 and 167 U/L depending on who measured it.

Two practical notes. The upper limit in the criterion is the laboratory’s, not the patient’s own serum LDH; substituting one for the other is a common slip. And meeting any one of the three criteria suffices, which is why the set is 98% sensitive and only about 77% specific — diuresis concentrates both LDH and protein in the residual fluid, and is the usual reason a genuine transudate is labelled an exudate.

Frequently asked questions

How do I convert LDH from U/L to µkat/L?

Divide by 60, or multiply by 0.0166667. This is a definition, not a measurement: one unit is one micromole per minute and one katal is one mole per second, so a pleural fluid LDH of 320 U/L is 5.333 µkat/L. µmol/s/L is the same quantity as µkat/L and needs no conversion between them.

Is U/L the same as IU/L for LDH?

Yes, exactly. The ‘international’ refers to the internationally agreed definition of the enzyme unit — one micromole per minute — rather than to a separate potency standard, so the two spellings denote the same quantity and no arithmetic is needed between them.

Can LDH be converted to a molar concentration?

No. LDH is reported as catalytic activity — a rate, not an amount of substance — so there is no molar concentration for it to convert to. The enzyme’s own molecular weight of about 144 kDa is irrelevant here, and the site’s factor table marks lactate dehydrogenase as an activity measurand for exactly this reason.

What pleural fluid LDH means exudate?

Light’s criteria give two independent LDH tests: a fluid-to-serum LDH ratio above 0.6, and a fluid LDH above two-thirds of the laboratory’s own upper limit of normal for serum LDH. Meeting either suffices. With the published upper limits on this page that second threshold works out between 143 and 167 U/L, so there is no single correct absolute number — which is the point of the wording.

Why do laboratories not publish an LDH reference interval?

Because the result depends on the method. Mayo returns an interpretive report for body fluid LDH rather than a numeric interval, and ARUP lists serum LDH as ‘by report; reports may vary based on instrumentation’. One laboratory that does publish numbers also records that its current method reads about 45% lower than its previous one.

Related calculators

References

Criteria for identifying exudative pleural effusions. MSD Manual, Professional Edition; accessed October 2026. Tabulates Light’s three criteria with their individual sensitivities and specificities.
Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507-13.
Exeter Clinical Laboratory International. Lactate dehydrogenase (LDH) — serum reference ranges effective 15 December 2020, with the note that the current method reads about 45% lower than the previous one. Test directory; accessed October 2026.
Awanui Labs. Changes to lactate dehydrogenase reference intervals, effective 18 February 2023. Referrer update; accessed October 2026.
Bureau International des Poids et Mesures. The International System of Units (SI), 9th ed. The katal, mol/s, as the SI unit of catalytic activity; 1 U = 1 umol/min, so 1 U/L = 1/60 ukat/L by definition.

Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/