A 28-year-old man of Bombay blood group phenotype requires an emergency transfusion following a motor vehicle accident. His blood type is negative for A, B, and H antigens. Which investigation is most appropriate to confirm the diagnosis of Bombay blood group (hh phenotype)?

## Confirmatory Investigation for Bombay Blood Group Phenotype **Key Point:** Bombay blood group (hh phenotype) is characterized by the absence of H antigen on red cells due to deficiency of the FUT1 enzyme (α-1,2-fucosyltransferase). The saliva inhibition test is the classic confirmatory investigation. ### Why Saliva Inhibition Test is Correct The saliva inhibition test is the gold standard for confirming Bombay phenotype: 1. **Principle:** H antigen is a precursor for A and B antigens. In Bombay individuals, the FUT1 gene is non-functional, so H antigen is NOT synthesized on RBCs or secreted in saliva. 2. **Procedure:** Patient's saliva is incubated with anti-H lectin (from *Ulex europaeus*), which binds to H antigen if present. The mixture is then added to O group RBCs. 3. **Expected result in Bombay phenotype:** Anti-H lectin is NOT inhibited by saliva (no H antigen present), so it reacts with O RBCs → **agglutination occurs**. 4. **Contrast with normal secretors:** Normal secretors have H antigen in saliva → anti-H lectin is inhibited → no agglutination. **High-Yield:** Absence of inhibition (positive agglutination) confirms Bombay phenotype. This is a bedside-friendly, cost-effective test widely available in Indian blood banks. ### Comparison of Investigative Approaches | Investigation | Principle | Utility | Limitation | |---|---|---|---| | **Saliva inhibition test** | H antigen in saliva inhibits anti-H lectin | Confirms Bombay phenotype; cheap, rapid | Requires patient cooperation; non-secretors may give false results | | **Anti-H monoclonal antibody** | Direct RBC testing | Confirms absence of H antigen on RBCs | Does NOT distinguish Bombay from rare non-secretor phenotypes | | **Absorption-elution** | Detects A/B antigens on RBCs | Confirms absence of A/B antigens | Indirect; does NOT specifically identify Bombay | | **FUT1 gene mutation** | Molecular genotyping | Definitive genetic confirmation | Expensive; not routine; requires specialized lab | **Clinical Pearl:** Bombay individuals have naturally occurring anti-H, anti-A, and anti-B antibodies in their serum. They can ONLY receive blood from other Bombay donors—transfusion from O group RBCs will cause a hemolytic reaction. **Mnemonic:** **BOMBAY = No H, No A, No B on RBCs** — but they have anti-H, anti-A, anti-B in serum (naturally occurring IgM antibodies). The saliva inhibition test is the **classic confirmatory test** because it detects the absence of H antigen secretion. ### Diagnostic Algorithm for Suspected Bombay Phenotype ```mermaid flowchart TD A[RBC typing: Negative for A, B, H antigens]:::outcome --> B[Perform saliva inhibition test]:::action B --> C{Agglutination with O RBCs?}:::decision C -->|Yes, agglutination| D[H antigen NOT in saliva]:::outcome C -->|No agglutination| E[H antigen present in saliva]:::outcome D --> F[Bombay phenotype confirmed]:::action E --> F2[Non-Bombay phenotype]:::action F --> G[Transfuse only Bombay blood]:::urgent ``` [cite:Harmening 6e Ch 4]