Why serum LDH is increase

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LDH lactate dehydrogenase isoenzymes diagram causes elevation

Here is a thorough explanation of why serum LDH is elevated, drawn from authoritative textbook sources:

Why Is Serum LDH Elevated?

What Is LDH?

Lactate dehydrogenase (LDH) is an enzyme present in virtually all cells of the body. The highest concentrations are found in the liver, muscle, and kidney; moderate concentrations are found in red blood cells. Because it is intracellular, LDH leaks into the bloodstream whenever cells are damaged or destroyed - making it a nonspecific but sensitive marker of tissue injury. - Robbins & Kumar Basic Pathology

The normal serum reference range is approximately 122-222 U/L (varies by laboratory).

Main Causes of Elevated Serum LDH

1. Hemolytic Anemia

Red blood cells contain large amounts of LDH. When RBCs are destroyed prematurely (hemolysis), LDH is released into the serum. Classic lab findings include:

High LDH
Low haptoglobin
Elevated indirect bilirubin
Elevated reticulocyte count
Schistocytes or spherocytes on blood smear

Causes include autoimmune hemolytic anemia, sickle cell disease, G6PD deficiency, microangiopathic hemolytic anemia (TTP, HUS), and mechanical hemolysis (prosthetic valves). - Harrison's Principles of Internal Medicine 22E

2. Myocardial Infarction (Heart Attack)

Cardiomyocytes are rich in LDH. Necrosis of heart muscle during MI causes massive LDH release. LDH1 isoenzyme (cardiac specific) rises 24-48 hours after infarction and peaks at 3-6 days, remaining elevated longer than troponin or CK-MB.

3. Liver Disease

The liver is one of the richest sources of LDH. Hepatocellular damage from hepatitis, cirrhosis, or ischemic hepatitis causes LDH release. Extremely elevated LDH (often >5x normal) signals massive hepatocyte damage - typically from ischemia ("shock liver") or drug toxicity such as acetaminophen overdose. - Miller's Anesthesia, 10e

4. Malignancies (Cancer)

LDH elevation is a hallmark of many cancers because:

Rapid tumor cell turnover and necrosis
Tumor cells preferentially use anaerobic glycolysis (Warburg effect), producing more LDH
Malignant infiltration of the liver causes LDH to rise alongside alkaline phosphatase
Cancers strongly associated with high LDH include: lymphoma, leukemia, metastatic melanoma, germ cell tumors, and lung cancer
LDH is used as a prognostic marker in NHL, multiple myeloma, and AJCC staging of melanoma

5. Pulmonary Embolism (PE)

Pulmonary infarction from a large PE causes lung tissue death and LDH release. The classic triad in PE (though rarely all present together) was LDH + elevated bilirubin + normal AST.

6. Rhabdomyolysis (Muscle Breakdown)

Skeletal muscle injury - from trauma, crush injuries, prolonged seizures, extreme exercise, myositis, or drugs (statins, alcohol) - liberates massive amounts of LDH along with CK and myoglobin. - Miller's Anesthesia, 10e

7. Renal Infarction

Renal tissue necrosis (e.g., from renal artery occlusion) causes LDH release due to the kidney's high LDH content.

8. Acute Stroke

Brain tissue ischemia or infarction can release LDH into the circulation.

9. Megaloblastic Anemia

In B12 or folate deficiency, ineffective erythropoiesis causes intramedullary hemolysis (destruction of RBC precursors in the bone marrow before they even enter the circulation), leading to elevated LDH.

10. Pancreatitis / Bowel Infarction

Tissue necrosis anywhere in the body - including acute pancreatitis or intestinal ischemia - can raise LDH.

LDH Isoenzymes - Clinical Significance

LDH has 5 isoenzymes (LDH1-LDH5) with different tissue distributions, helping localize the source:

Isoenzyme	Predominant Source	Clinical Clue
LDH1	Heart, RBCs, kidney	Elevated in MI, hemolysis
LDH2	Heart, RBCs	Normally highest in serum
LDH3	Lung, spleen, pancreas	Elevated in PE, pancreatitis
LDH4	Placenta, liver, muscle
LDH5	Liver, skeletal muscle	Elevated in liver disease, rhabdomyolysis

In MI, the LDH1/LDH2 ratio flips ("LDH flip") - LDH1 exceeds LDH2.

Key Point: Specificity vs. Sensitivity

LDH is a highly sensitive but non-specific marker. It is elevated in a wide variety of conditions, so it should always be interpreted in clinical context alongside:

AST, ALT, bilirubin (liver)
CK, troponin (muscle/heart)
Haptoglobin, reticulocyte count (hemolysis)
Alkaline phosphatase (malignancy vs. bone disease)

"Elevated LDH concomitant with elevated alkaline phosphatase suggests malignant infiltration of the liver." - Miller's Anesthesia, 10e

In summary, elevated serum LDH = cell damage somewhere in the body. The pattern of isoenzymes and co-existing lab abnormalities helps identify which organ system is involved.

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