Introduction
Lactate dehydrogenase, or LDH, is a marker of cell breakdown almost anywhere in the body, most notably red blood cells, liver, lungs, heart, brain, and kidneys. Thus, a negative LDH is an extremely useful test result because it tells you that all sorts of things are not going on with your patient.
An elevated LDH is also a useful and specific test result as long as you take a minute or two to evaluate the patient and to perform, if necessary, a few quick and relatively cheap laboratory studies.
LDH Basics
LDH is frequently encountered both clinically and on standardized tests to:
- Diagnose an exudative pleural effusion (the test is positive for exudate if the pleural fluid to serum LDH ratio is greater than 0.6 or if the pleural fluid LDH is more than two-thirds of the upper limit of normal)
- Rule out hemolysis
- Rule in ischemic hepatitis (in a patient with marked hypertransaminasemia, a very high LDH supports the diagnosis of ischemic hepatitis, or “shock liver,” if LDH/ALT>1)
- Rule out Pneumocystis jiroveci pneumonia in immunosuppressed patients
- Diagnose renal infarction (LDH which is four times the upper limit of normal is supportive of this diagnosis)
- Diagnose germ cell tumors
- Predict mortality in patients with acute pancreatitis
- Diagnose acute coronary syndrome (a long time ago!)
A More Advanced Concept
Let’s say a patient with chronic kidney disease presents with vague flank pain. The urinalysis is unconvincing and the CT of the abdomen and pelvic without contrast is negative. You suspect acute renal infarction but you are not sure whether you want to perform a CT with contrast because of the risk of contrast nephropathy. So you get an LDH – and it’s high.
You still don’t know if you want to go ahead with the CT with contrast because the LDH could be coming from anywhere. So what do you do?
The next step is to rule out alternative causes of elevated LDH. The workup, with results, might look something like this:
- Brain: clinically negative for stroke
- Heart: no chest pain or shortness of breath, and/or negative troponins and electrocardiogram
- Blood: negative hemolysis workup (CBC, peripheral blood smear, reticulocyte count, bilirubin, haptoglobin)
- Liver: negative liver chemistry studies
- Skeletal muscle: negative creatine kinase (CK)
Now the post-test probability of acute renal infarction is much higher because alternative causes of elevated LDH have been ruled out. So you decide to go ahead with the CT abdomen and pelvis with contrast.
LDH – get it!
References
- Desai, Samir P., MD, Clinician’s Guide to Laboratory Medicine: Pocket (2009, reviewed here)
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