The lactate dehydrogenase (LDH) test is an important blood test that provides vital information about the health of body tissues. LDH enzyme levels in the bloodstream serve as a useful biomarker in detecting tissue damage from injury, disease, infection or cancer. 

What is LDH? 

Lactate dehydrogenase or LDH is an intracellular enzyme present in all major organs. 

• There are five isoforms of LDH enzymes, each coded by different genes and showing variable distribution across body tissues. 
• When cell injury or death occurs due to damage, medical conditions or inflammation, intracellular LDH is released into the extracellular fluid and blood circulation. 

What is the LDH Test?

The lactate dehydrogenase test, commonly referred to as the LDH test or LD test, is a diagnostic blood test that determines the level of lactate dehydrogenase enzymes circulating in the blood plasma. 

• It measures the collective activity of the five LDH isoenzymes to provide an indirect estimate of tissue breakdown in health disorders associated with cell death. 
• By assessing the rise in extracellular LDH released from cells, the LDH blood test effectively detects cellular damage and tissue breakdown in a wide spectrum of cardiovascular, liver, bone, infectious, neoplastic and hematologic diseases. 

Purpose of Lactate Dehydrogenase Test

Some major purposes of measuring blood LDH levels through testing include:

1. Detecting and evaluating tissue injury: 

• Significantly elevated LDH indicates tissue damage from myocardial infarction, acute liver failure, extensive burns, haemolysis, muscular dystrophy, sepsis or other medical issues causing cell death. 
• It confirms diagnosis, assesses severity and monitors disease courses. 

2. Diagnosing infections and inflammation:

• Increased LDH occurring with bacterial infections (meningitis, encephalitis), viral infections (mononucleosis, cytomegalovirus), and inflammation of joints, blood vessels, or heart tissue signals tissue injury.  

3. Cancer screening and treatment monitoring:

• Many cancer cells have higher LDH expression. 
• Elevated blood LDH levels diagnose certain cancers (lymphoma, seminoma, testicular cancer). 
• Serial LDH measurements during chemotherapy assess tumour response and check for recurrence or progression in cancers like lymphoma, melanoma and germ cell tumours. 

When is the LDH Test Ordered?

Doctors order the LDH blood test when symptoms indicate:

• Chest pain, Heart attack, Angina, Heart failure
• Hepatitis, Jaundice, Cirrhosis
• Acute kidney injury, Glomerulonephritis 
• Pneumonia, Pulmonary embolism  
• Anaemia, Leukemias, Lymphomas  
• Muscular dystrophy, Myositis  
• Meningitis, Encephalitis, Brain injury
• Sepsis, Abscesses, Mononucleosis  
• Lymphoma, Myeloma, Melanoma  

What Happens During the LDH Test? 

The LDH test involves a simple blood draw, also called venipuncture. Here are the steps:

1. A tourniquet is wrapped around the upper arm to make the veins below swell with blood.
2. Using a sterile, disposable needle attached to a syringe, around 2-3 mL of blood is withdrawn and collected in a tube coated with clot activators. 
3. Once sufficient samples are collected, applying a cotton swab and maintaining pressure for 5 minutes will stop any further bleeding or oozing at the needle puncture site.

Uses of LDH Test in Healthcare

Measuring blood LDH levels has many clinical applications including:  

1. Detecting cardiac injury in myocardial infarction:  

• LDH rises 12 hours after a heart attack, peaks in 2-3 days and returns to baseline by 5 to 10 days. 
• Its rise and gradual fall help diagnose and monitor heart damage.

2. Evaluating liver disease and hepatitis: Markedly elevated LDH indicates infectious hepatitis and acute liver necrosis diagnosed by liver biopsy.  

3. Identifying respiratory disorders: This test helps identify viral pneumonia where alveolar wall necrosis releases LDH into circulation. 

4. Diagnosing primary and metastatic brain cancers: Cancers that increase vascular permeability, letting tissue LDH enter the blood, can also be detected through this test. 

5. Predicting complications: This test can be helpful in preventing complications like sepsis, shock, and multiorgan failure caused by increased LDH in critically ill patients.

LDH Test Procedure

The step-by-step LDH testing process involves:   

1. Collection:

• About 2.5 mL whole blood is collected by venipuncture using a sterile needle. 
• After centrifugation, the separated plasma is analysed immediately or stored at 39°F-46°F (4°C-8°C).  

2. Assay Methodology:

• Most laboratories now use automated clinical chemistry analyzers that determine LDH activity by spectrophotometric assay methods. 
• LDH catalyses the reduction of pyruvate utilising NADH whose decreasing concentration is measured as a decline in absorbance at 339 nm providing an indirect quantification of LDH activity.

3. Reference Range Interpretation: 

• Measured LDH values are interpreted by comparing against a reference interval, dividing normal from abnormal results. 
• Adult reference ranges differ between males and females as:
  • Males = 135-225 U/L
  • Females = 135-214 U/L

How Painful is the LDH Test?

As stated earlier, the LDH test simply requires 2-3 mL of blood collected through venipuncture from a vein in the arm, which may feel like a quick needle prick. 

Using distraction techniques, numbing medication, relaxation methods, and paediatric nitrous oxide can ease this short episode of sharp sensation further. Typically, the test is very comfortable, and most patients do not experience any pain or discomfort during or after the procedure.  

How to Prepare for the LDH Test?

Generally, no special preparation is required before the LDH test. Here’s what to expect and do before the procedure: 

• No fasting is required prior as eating does not affect results. 
• Avoid exhausting muscular activity a day before the test, which can falsely raise LDH levels temporarily. 
• Inform your doctor about any medications, supplements or herbal products you are taking currently.
• Discontinue Vitamin C supplements at least 9-12 hours before the blood draw, as they can alter test accuracy. 
• Wear comfortable upper garments with easily rollable sleeves to access the inner elbow region smoothly for sampling. 

What Does LDH Test Result Mean? 

LDH test reports provide a measurement of your blood lactate dehydrogenase enzyme level along with standard reference intervals for comparison marked as “Normal”, “Low” or “High”.

1. Normal LDH level: 

• A normal result ranging 140-280 units/L signifies no significant tissue injury or cell death. 
• It is a negative or normal test. 

2. Elevated LDH level:

• Higher than normal LDH indicates cellular damage from diseases like sepsis, blood cancer or muscle trauma, releasing intracellular enzymes into the bloodstream. 
• Levels above 500 units/L confirm abnormal tissue destruction requiring further evaluation. 
• Really high >1500 units/L suggests massive cellular necrosis as in extensive burns, haemolysis or advanced cancers.

3. Low LDH levels: 

• Readings below the reference are biologically insignificant medically. 
• Technical errors during analysis or sample collection may contribute to falsely lowered values. 
• Otherwise, chronically low LDH levels could reflect malnutrition or chronic alcoholism. 

Conclusion

The LDH or lactate dehydrogenase test effectively detects tissue damage helping diagnose heart disease, liver disease, cancer, infections, muscular disorders, and other medical conditions. Tracking LDH levels regularly serves as an important biomarker to monitor disease progression and treatment response in cancer patients.

FAQs

1. What is a normal LDH level?

Ans: A normal LDH level ranges from 140 to 280 units/litre (U/L) in the blood. However, the reference range may differ across laboratories. 

2. What happens if the LDH test is positive?  

Ans: A positive LDH test means your LDH level is above the normal range. An elevated LDH indicates tissue or cell damage caused by conditions like heart disease, liver disease, cancer, infection, injury or muscle damage. 

3. What happens if the LDH test is negative?

Ans: A negative LDH test means your LDH level is within the normal 140-280 U/L range, indicating no significant tissue injury. It rules out the suspected medical condition. No further evaluation may be needed unless symptoms persist.

4. What are the complications of the LDH test?

Ans: The LDH test is a safe procedure. Rare complications include excessive bleeding, fainting, infection or clotting at the needle puncture site. Bruising of the skin may occur.   

5. How long does an LDH test take? 

Ans: The LDH test is performed quickly and takes about 15 minutes only. A healthcare provider draws a blood sample, which is sent to a laboratory for analysis. Digital test reports are available within a few hours or the next day.