Lactate Dehydrogenase (LDH) Unit Converter
Lactate Dehydrogenase (LDH) Unit Converter
Convert LDH between U/L, IU/L, µkat/L and nkat/L — and start with the fact that haemolysing the sample raises the result all by itself.
Lactate Dehydrogenase (LDH) converter
Activity unitsSerum LDH 200 U/L
Formula and conversion factor
U/L = µkat/L × 60
nkat/L = U/L × 16.667
- U
- 1 unit = 1 µmol of substrate converted per minute
- kat
- 1 katal = 1 mol per second, so 1 U/L = 1/60 µkat/L; IU/L is numerically identical to U/L
- no molar unit
- LDH is reported as catalytic activity, not as a mass or an amount of enzyme, so no µmol/L conversion exists
- method dependence
- activity depends on substrate direction, buffer and temperature, so intervals are not transferable between laboratories and neither are serial results across a change of analyser
Worked example
Serum LDH 200 U/L
200 ÷ 60 = 3.333 µkat/L
= 3,333 nkat/L = 200 IU/L
Within the adult reference interval of 140–280 U/L, or 2.333–4.667 µkat/L
Reference interval in both conventions
| U/L | µkat/L | |
|---|---|---|
| Adult reference interval | 140 – 280 | 2.333 – 4.667 |
| Above the interval | > 280 | > 4.667 |
| Pleural fluid LDH above two-thirds of the serum upper limit (Light’s criteria) | > 187 | > 3.111 |
Working through a raised LDH
| Ask | Because |
|---|---|
| Was the sample haemolysed? | Red cells are rich in LDH and release it on lysis, so a difficult venepuncture raises the result with no disease present. This is the commonest cause of an unexpected high LDH |
| Is there haemolysis in the patient? | Intravascular haemolysis raises LDH with a low haptoglobin, a raised unconjugated bilirubin and a reticulocytosis — the four are read together, never LDH alone |
| Is there tissue injury elsewhere? | LDH is released by liver, cardiac and skeletal muscle, lung, kidney and marrow; hepatitis, infarction, rhabdomyolysis and pulmonary embolism all raise it |
| Is there a tumour or a high cell turnover? | Lymphoma, germ cell tumours and tumour lysis raise LDH, and it is used prognostically in several — but it identifies no tumour type on its own |
| Is this a pleural or ascitic fluid? | Fluid LDH is one of Light’s criteria and is one of the few settings where LDH answers a specific question |
A test that is only useful next to another test
Lactate dehydrogenase is reported as catalytic activity, in U/L or the numerically identical IU/L, and in µkat/L or nkat/L where SI units are used. A unit is one micromole of substrate converted per minute and a katal is one mole per second, so the entire conversion is a division by 60: an LDH of 200 U/L is 3.333 µkat/L. Nothing about the enzyme molecule enters the arithmetic, which is why no molar figure is offered. Reference intervals move substantially with method — 140–280 U/L is the common convention, while some laboratories quote 122–222 U/L — so serial results should not be compared across a change of analyser.
The single most useful thing to know about a high LDH is that the sample may have caused it. Red cells contain far more LDH than plasma does, and any haemolysis during collection or transport releases it, producing an artefactual rise in a patient with nothing wrong. A difficult venepuncture, a narrow needle, vigorous mixing or a delayed and shaken specimen will all do it. Before working up an isolated raised LDH, look at the haemolysis index on the report or simply repeat the sample.
Beyond that, LDH is spectacularly non-specific. The enzyme is present in essentially every nucleated cell, so hepatitis, myocardial and skeletal muscle injury, pulmonary embolism, renal infarction, megaloblastic anaemia, marrow failure and a wide range of tumours all raise it. Isoenzyme fractionation, which could localise the source, is rarely available and rarely requested. A raised LDH therefore points somewhere without saying where, and a tissue-specific test — ALT, creatine kinase, troponin — has to follow it.
Two settings do give LDH a defined job. In suspected haemolysis it is read as a set with haptoglobin, unconjugated bilirubin and the reticulocyte count, where a raised LDH with a suppressed haptoglobin is informative in a way that either alone is not. And in pleural fluid it is one of Light’s criteria, separating exudate from transudate against two-thirds of the serum upper limit. In oncology it carries prognostic weight in lymphoma, germ cell tumours and melanoma, and rises in tumour lysis syndrome.
Frequently asked questions
How do I convert LDH from U/L to µkat/L?
Divide by 60, because a unit is one micromole per minute and a katal is one mole per second. An LDH of 200 U/L is 3.333 µkat/L, or 3,333 nkat/L. IU/L and U/L are the same number.
Can a haemolysed sample raise the LDH?
Yes, and it is the commonest cause of an unexpected high result. Red cells are rich in LDH and release it when they lyse during a difficult venepuncture or in transport. Check the haemolysis index on the report, or repeat the sample, before investigating further.
Why is LDH considered non-specific?
The enzyme is present in essentially every nucleated cell, so liver, cardiac and skeletal muscle, lung, kidney, marrow and tumour tissue all release it. Isoenzyme fractionation could localise the source but is rarely available, so a raised LDH points somewhere without saying where.
What is LDH actually useful for?
As part of a defined set. With haptoglobin, unconjugated bilirubin and the reticulocyte count it supports a diagnosis of haemolysis; in pleural fluid it is one of Light’s criteria; and it carries prognostic weight in lymphoma, germ cell tumours and melanoma.
Do LDH reference intervals differ between laboratories?
Considerably. Activity depends on the substrate direction, buffer and temperature the method uses, so 140–280 U/L and 122–222 U/L are both in current use. Serial results should not be compared across a change of analyser.
Related calculators
References
- Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507–513.
- Lippi G, Salvagno GL, Montagnana M, Brocco G, Guidi GC. Influence of hemolysis on routine clinical chemistry testing. Clin Chem Lab Med. 2006;44(3):311–316.
- IFCC primary reference procedure for the measurement of catalytic activity concentration of lactate dehydrogenase at 37°C.