Indirect Coombs Test
HemogramaÚltima revisão: 7 de abril de 2026. Abordagem de fontes: contexto padrão de interpretação laboratorial, material médico de referência e orientações clínicas ou de saúde pública quando relevantes.
O que é Indirect Coombs Test?
The indirect Coombs test, also known as the indirect antiglobulin test (IAT), detects free (unbound) antibodies circulating in the patient's serum or plasma that have the potential to bind to red blood cell (RBC) antigens. The test is performed by incubating the patient's serum with reagent red blood cells of known antigen composition. If antibodies in the serum bind to antigens on the reagent RBCs, subsequent addition of anti-human globulin (AHG) causes agglutination, indicating a positive result.
The IAT is fundamental to pretransfusion compatibility testing (crossmatching) and prenatal antibody screening. In the blood bank, it ensures that donor red blood cells are compatible with the recipient's serum before transfusion, preventing potentially fatal hemolytic transfusion reactions. In obstetrics, it screens pregnant women for clinically significant RBC alloantibodies—particularly anti-D (Rh) antibodies—that can cross the placenta and cause hemolytic disease of the fetus and newborn (HDFN). The IAT also plays a role in antibody identification panels when unexpected antibodies are detected.
Por que isso importa
The indirect Coombs test is critical for safe blood transfusion and prenatal care. In transfusion medicine, a positive IAT (antibody screen) means the patient has developed alloantibodies against foreign RBC antigens, likely from prior transfusion or pregnancy. These antibodies must be identified so that antigen-negative blood can be provided, preventing hemolytic transfusion reactions. In pregnancy, detection of anti-D or other clinically significant antibodies allows monitoring and intervention (anti-D immunoglobulin, intrauterine transfusion) to prevent or manage hemolytic disease of the newborn.
Faixas de referência normais
| Grupo | Faixa | Unidade |
|---|---|---|
| Adults and children | Negative | (no agglutination) |
As faixas de referência podem variar entre laboratórios. Sempre compare seus resultados com as faixas fornecidas pelo seu local de exame.
O que significam níveis altos de IAT
Causas comuns
- RBC alloantibodies from prior transfusion (anti-K, anti-E, anti-Fy-a, anti-Jk-a, etc.)
- Anti-D antibodies in Rh-negative individuals sensitized by Rh-positive blood
- Naturally occurring antibodies (anti-A, anti-B in ABO system)
- Warm or cold autoantibodies reacting with screening cells
- Passively acquired antibodies (anti-D immunoglobulin, IVIG)
Possíveis sintomas
- No direct symptoms from a positive IAT
- If antibodies cause hemolysis after transfusion: fever, chills, back pain, dark urine, hypotension
- In pregnancy: fetal anemia, hydrops fetalis, neonatal jaundice (due to HDFN)
O que fazer: A positive IAT triggers antibody identification using panels of reagent red blood cells with known antigen profiles. Once the antibody specificity is determined, antigen-negative compatible blood is selected for transfusion. In pregnancy, antibody titers are monitored serially—rising titers (typically critical titer ≥16 for anti-D) prompt referral for fetal surveillance including middle cerebral artery Doppler ultrasound. Rh-negative women receive anti-D immunoglobulin prophylaxis at 28 weeks and after delivery of an Rh-positive infant.
O que significam níveis baixos de IAT
Causas comuns
- Normal result—no clinically significant RBC antibodies detected in serum
Possíveis sintomas
- No symptoms associated with a negative IAT
O que fazer: A negative IAT means no unexpected RBC antibodies were detected, and crossmatch-compatible blood can be issued for transfusion. In pregnancy, a negative antibody screen is reassuring but should be repeated at 28 weeks gestation and after delivery. Rh-negative women with a negative screen should still receive anti-D immunoglobulin prophylaxis.
Quando o exame de IAT é recomendado?
- Pretransfusion compatibility testing (type and screen)
- Prenatal antibody screening at first visit and 28 weeks
- Evaluation after suspected hemolytic transfusion reaction
- Monitoring antibody titers in alloimmunized pregnancies
- Prior to Rh immunoglobulin administration
Perguntas frequentes
Biomarcadores relacionados
Leitura relacionada
Condições
Referências e abordagem de revisão
As páginas do glossário de biomarcadores são explicações educativas e devem ser interpretadas junto com as faixas de referência e observações fornecidas pelo seu laboratório e pelo seu médico. Para conhecer nossos padrões editoriais e processo de revisão, veja nossa Política editorial e a nossa revisão de conteúdo.
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Enviar resultados de exames →Aviso médico: Estas informações são apenas educativas e não substituem orientação, diagnóstico ou tratamento médico profissional. As faixas de referência podem variar entre laboratórios. Sempre converse com seu profissional de saúde sobre a interpretação dos seus resultados específicos.