LDHisoBody

SydPath Information Sheet	Dr Graham Jones Department of Chemical Pathology

LD Isoenzymes

Physiology Lactacte Dehydrogeanse (LD) is an enzyme made up of 4 subunits Each of these subunits may be of the H (heart) or M (muscle) types. The composition of each of the subtypes (LD 1 to 5) is as follows: LD1- HHHH, LD2 - HHHM, LD3 - HHMM, LD4 - HMMM, LD5 - MMMM. The isoenzymes have varying half lives in the circulation from T1/2 100hrs fro LD1 to 10 hours for LD5. In normal serum the most prominent form LDH is LD2 although all types are normally present. LD is found in most cell types in the body with Isoenzymes LD1 and LD2 found in high concentrations in red blood cells and cardiac myocytes, and LD5 in muscle cells and hepatocytes. Pathology Raised levels of the various LD isenzymes generally reflects damage to tissues containing high concentrations of that isoenzyme with subsequent release into the circulation. Thus myocardial infarction or red cell destruction leads to an increase in circulating concentrations of LD1 and, to a lesser extent LD2 and damage to skeletal muscle or hepatocytes is marked by an increase in LD5. The table below shows possible causes of elevations. Causes of Elevated LD Isoenzymes Predominant elevation of LD1 Haemolysis (in-vivo or in-vitro) Ineffective erthryopoiesis (eg vitamin B12 or folate deficiency) Myocardial infarction or other cardiac damage (eg trauma, cardiac surgery) Malignancies (childhood ALL, Wilm's Tumour, Germ cell Carcinoma) Renal infarct Predominant elevation of LD5 Rhambdomyolysis (any cause, CK and AST also elevated) Hepatitis (any cause, ALT and AST also elevated) Predominant elevation of LD2-4 Lymphoma, leukaemia Infectious mononucleosis (together with LD 5) Damage to spleen, lungs, lymph nodes, platelets Mixed pattern (all isoenzymes) Interpretation uncertain. Usually does not add to clinical interpretation. Unusual patterns Macro LD (LD-antibody complex, marked by missing, extra, or abnormal bands) Congential deficiency of M or H (not associated with clinical effects) Clinical Use LD isoenzymes are of use in evaluating the cause of an unknown elevation of LD. Often the clinical setting and other tests can answer the question more directly, eg elevated CK in rhabdomyolysis and other tests in haemolysis. It not appropriate to measure LD isoenzymes if the total LD activity is not raised above the reference interval. LD Isoenzyme reporting at SydPath This test is referred to an external laboratory. In addition to fractionating the LD which is reproted as a percetage of the total LD activity, a repeat total LD measurement is performed. Due to methodological differences the total LD result associated with the fractionated results is numerically lower than the usual SydPath result by a factor of about 2. this is also reflected in the different reference interval use for this result.

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The Pathology Service of St Vincent's Hospital, Sydney Under the Care of the Sisters of Charity
Last updated 28/01/05