ANTI-D (Rh) Antibody Titre – Serum Test Overview
Introduction
The Anti-D (Rh) antibody titre test is a serological investigation that measures the concentration (titre) of anti-D antibodies in a patient's serum. These antibodies are formed against the D antigen of the Rhesus (Rh) blood group system, most commonly in Rh-negative individuals who have been exposed to Rh-positive red blood cells.

This test is crucial in obstetric care, particularly for Rh-negative pregnant women, as it helps monitor the risk of hemolytic disease of the fetus and newborn (HDFN).

Purpose of the Test
To detect and quantify the level of anti-D antibodies in the serum of Rh-negative individuals.

To monitor Rh-negative pregnant women with potential sensitization to Rh-positive fetal blood.

To assess the need for interventions such as intrauterine transfusion or early delivery.

To evaluate the effectiveness of anti-D immunoglobulin prophylaxis.

Clinical Background
When an Rh-negative person is exposed to Rh-positive red cells, typically through pregnancy, blood transfusion, or fetomaternal hemorrhage, their immune system may develop anti-D antibodies.

In pregnancy, if a sensitized Rh-negative mother carries an Rh-positive fetus, maternal anti-D antibodies can cross the placenta and attack fetal red blood cells, causing hemolytic anemia, jaundice, or even fetal hydrops.

Sample Type
Specimen: Serum

Collection: Venous blood is drawn into a plain or serum-separator tube (SST). After clotting and centrifugation, serum is separated for testing.

Test Methodology
Indirect Antiglobulin Test (IAT) is typically used.

The serum is serially diluted, and each dilution is tested against Rh-positive red blood cells.

The highest dilution of serum that still causes visible agglutination is reported as the antibody titre.

Titres are expressed as ratios (e.g., 1:8, 1:16, 1:32, etc.).

Interpretation of Results
Negative Titre: No detectable anti-D antibodies; indicates no sensitization.

Low Titre (e.g., ≤1:8 or ≤1:16): Generally not associated with significant fetal risk.

Critical Titre (usually ≥1:16 to 1:32): Suggests a potential risk of HDFN; warrants close fetal monitoring via Doppler ultrasound (e.g., middle cerebral artery peak systolic velocity).

Rising Titre: Indicates increasing antibody levels and a growing risk to the fetus.

Note: The critical titre threshold may vary by laboratory or institution, but 1:16 is commonly used as a reference point.

Clinical Applications
Prenatal Screening: Routine testing for Rh-negative pregnant women.

Monitoring Alloimmunized Pregnancies: Serial titres are used to assess disease progression.

Postnatal Management: Helps guide decisions regarding neonatal care (e.g., phototherapy, exchange transfusion).

Limitations
Cannot differentiate between IgG subclasses (some cause more severe hemolysis than others).

Not a direct measure of fetal hemolysis—must be interpreted alongside ultrasound findings, fetal monitoring, and amniocentesis, if necessary.

Conclusion
The anti-D (Rh) antibody titre is a valuable and non-invasive tool in obstetric immunohematology, particularly for managing Rh incompatibility in pregnancy. Its early and accurate interpretation can significantly reduce fetal morbidity and mortality associated with Rh isoimmunization.