What is a normal lactate dehydrogenase (LDH) level?

Normal Lactate Dehydrogenase (LDH) Levels

The normal range for serum lactate dehydrogenase (LDH) is typically between 313-618 IU/L, though reference ranges may vary slightly between laboratories. 1

Understanding LDH

LDH is an enzyme found in nearly all cells of the body and is released into the bloodstream when cells are damaged. It exists in five different isoenzymes, each predominantly found in different tissues:

• LDH1 & LDH2: Heart, red blood cells, kidneys
• LDH3: Lungs, lymphatic tissue
• LDH4: Placenta, kidneys
• LDH5: Liver, skeletal muscle

Clinical Significance of LDH Levels

Normal Range Interpretation

• Values within the normal range (313-618 IU/L) generally indicate absence of significant tissue damage
• Minor elevations may not be clinically significant

Elevated LDH Levels

Elevations in LDH can indicate various conditions:

• Mild elevation (up to 2× normal): Often nonspecific, may indicate mild tissue damage
• Moderate elevation (2-3× normal): Seen in 45% of cases with elevated LDH 2
• Significant elevation (3-10× normal): Seen in 47.5% of cases 2
• Marked elevation (>10× normal): Seen in 7.5% of cases, associated with poor prognosis (mortality >50%) 2

LDH in Specific Clinical Contexts

Pleural Fluid Analysis

• In pleural fluid analysis, LDH is a key component of Light's criteria for distinguishing exudates from transudates
• Pleural fluid is considered an exudate if:
  • Pleural fluid/serum LDH ratio >0.6
  • Pleural fluid LDH >2/3 the upper limit of normal for serum LDH 3
• In heart failure with exudative pleural effusion, pleural fluid LDH <250 U/L is a diagnostic criterion 3

Malignancies

• Elevated LDH (>900 IU/L) may suggest hematologic malignancies, particularly:
  • Acute lymphoblastic leukemia (ALL): Often markedly elevated (>900 IU/L in 78.9% of cases) 4
  • Non-Hodgkin's lymphoma: Moderately elevated (250-500 IU/L) 5
• Very high isolated LDH (≥800 IU/L) without liver enzyme elevation is strongly associated with:
  • Cancer (27% of cases)
  • Liver metastases (14%)
  • Hematologic malignancies (5%) 6

Prognostic Value

LDH has significant prognostic value in several conditions:

• Malignancies: Higher LDH levels correlate with poorer prognosis in lymphomas and melanoma 1, 5
• Very high LDH (>10× normal): Associated with:
  • ICU admission in 73% of cases
  • Mortality rate of 53% 2
• Very high isolated LDH (≥800 IU/L): Independent predictor of mortality (26.6% vs 4.3% in controls) 6

Monitoring Disease Progression

Serial LDH measurements are valuable for:

• Monitoring treatment response in malignancies
• Detecting disease progression or relapse
• Assessing treatment efficacy

Key Points to Remember

• A single elevated LDH value has limited diagnostic specificity
• LDH elevation should be interpreted in clinical context
• Serial measurements are more valuable than single readings
• Very high LDH levels warrant thorough investigation for underlying severe disease
• LDH isoenzyme analysis can help localize the source of tissue damage

When interpreting LDH levels, it's important to consider other laboratory values and clinical findings to determine the underlying cause of elevation and its clinical significance.