D-dimer

Overview of D-dimer Assays

Definition: D-dimer assays are quantitative tests used to measure the level of D-dimer in plasma. D-dimer is a specific fibrin degradation product (FDP) that results from the breakdown of cross-linked fibrin
Clinical Significance:
- Exclusion of Venous Thromboembolism (VTE): D-dimer assays are primarily used to rule out venous thromboembolism (deep vein thrombosis [DVT] and pulmonary embolism [PE]) in patients with a low or intermediate pretest probability
- Diagnosis of Disseminated Intravascular Coagulation (DIC): D-dimer is markedly elevated in DIC
- Monitoring Thrombolytic Therapy: Can be used to monitor the effectiveness of thrombolytic therapy
- Assessment of Other Conditions: Elevated D-dimer levels can also be seen in other conditions, including pregnancy, infection, inflammation, malignancy, and recent surgery
Limitations:
- Poor Specificity: Elevated D-dimer levels are not specific to VTE and can be seen in a variety of other conditions
- Age-Related Increase: D-dimer levels tend to increase with age
- Not Useful for Monitoring Anticoagulation: D-dimer is not used to monitor anticoagulant therapy

D-dimer assays are based on immunochemical methods that use monoclonal antibodies specific for D-dimer

Latex Agglutination Assay:
- Latex particles are coated with monoclonal antibodies against D-dimer
- When the reagent is mixed with a sample containing D-dimer, the antibodies bind to the D-dimer, causing the latex particles to agglutinate (clump together)
- The agglutination is measured turbidimetrically or nephelometrically, and the result is compared to a calibration curve
Enzyme-Linked Immunosorbent Assay (ELISA):
- Microplate wells are coated with monoclonal antibodies against D-dimer
- The sample is added to the wells, and any D-dimer present binds to the antibodies
- A secondary antibody conjugated to an enzyme is added, which binds to the D-dimer-antibody complex
- A substrate for the enzyme is added, and the resulting color change is measured spectrophotometrically
Immunoturbidimetric Assay:
- Antibodies against D-dimer are added to the sample
- As the antibodies bind to D-dimer, immune complexes form
- The turbidity of the solution is measured, which is proportional to the D-dimer concentration
Chemiluminescent Immunoassay:
- Similar to ELISA, but uses a chemiluminescent substrate instead of an enzyme-linked substrate
- The emitted light is measured, which is proportional to the D-dimer concentration

Sample Collection and Preparation:
- Collect blood in a sodium citrate (light blue top) tube
- Process the sample within the specified timeframe to obtain platelet-poor plasma (PPP)
- Follow the manufacturer’s instructions for sample storage and handling
Assay Performance:
- Follow the manufacturer’s instructions for the specific assay being used
- Ensure that all reagents are properly prepared and stored
- Load the samples and reagents onto the automated analyzer
- Start the assay and allow the instrument to perform the analysis
Quality Control:
- Run control materials at regular intervals (e.g., daily, with each batch of samples) to assess the accuracy and precision of the assay
- Evaluate control results and take corrective action if necessary
Result Calculation:
- The automated analyzer calculates the D-dimer concentration based on a calibration curve
- Ensure that the results are within the assay’s linear range

Negative D-dimer Result:
- D-dimer level is below the established cut-off value for the assay
- In patients with a low or intermediate pretest probability for VTE, a negative D-dimer result is used to rule out DVT and PE
Elevated D-dimer Result:
- D-dimer level is above the cut-off value
- Indicates the presence of fibrin degradation products in the plasma, suggesting that both coagulation and fibrinolysis have been activated
- Causes of Elevated D-dimer:
  - Venous Thromboembolism (VTE): DVT and PE
  - Disseminated Intravascular Coagulation (DIC)
  - Surgery
  - Trauma
  - Pregnancy
  - Malignancy
  - Infection
  - Inflammation
  - Advanced Age
- Note: An elevated D-dimer result is not specific for VTE, and further testing (e.g., imaging studies) is required to confirm the diagnosis

Pre-Analytical Variables:
- Improper Sample Collection:
  - Clotted sample: D-dimer cannot be accurately measured
  - Underfilled collection tube: Can affect the blood-to-anticoagulant ratio
- Delayed Processing:
  - Prolonged storage at room temperature can lead to falsely elevated D-dimer levels
  - Samples should be processed and tested as soon as possible or stored according to the manufacturer’s instructions
- Lipemia or Icterus:
  - Can interfere with optical measurements
Analytical Variables:
- Assay Type: Different D-dimer assays have different sensitivities and specificities
- Cut-off Values: Cut-off values for positivity can vary between laboratories
- Instrument Malfunction: Ensure proper calibration and maintenance of the analyzer
Patient-Related Variables:
- Age: D-dimer levels tend to increase with age
  - Age-Adjusted Cut-Offs: Some laboratories use age-adjusted D-dimer cut-off values to improve specificity in older adults
    - Age-Adjusted Cut-Off = 0.50 μg/mL + (Age x 0.01 μg/mL)
- Pregnancy: D-dimer levels are normally elevated during pregnancy
- Medical Conditions:
  - Inflammation
  - Infection
  - Malignancy
  - Liver Disease
  - Kidney Disease

D-dimer: A specific fibrin degradation product that is elevated in thrombotic conditions
Fibrinolysis: The enzymatic breakdown of fibrin clots
Venous Thromboembolism (VTE): A condition that includes deep vein thrombosis (DVT) and pulmonary embolism (PE)
Deep Vein Thrombosis (DVT): A blood clot in a deep vein, usually in the leg
Pulmonary Embolism (PE): A blood clot that travels to the lungs
Disseminated Intravascular Coagulation (DIC): A complex disorder with widespread clotting and bleeding
Sensitivity: The ability of a test to correctly identify individuals who have the condition (true positive rate)
Specificity: The ability of a test to correctly identify individuals who do not have the condition (true negative rate)