A 19-year-old girl is brought to the hospital 6 hours after alleged sexual assault. The medico-legal examination is in progress. To confirm the *presence* of seminal fluid and establish recent sexual contact, which is the most appropriate initial screening investigation?

## Initial Screening for Seminal Fluid in Sexual Assault Cases ### Acid Phosphatase Test — The Standard Initial Screening Investigation **Key Point:** The **acid phosphatase (AP) test** on vaginal swab is the most appropriate *initial screening* investigation to confirm the presence of seminal fluid in cases of alleged sexual assault. This is a well-established presumptive biochemical test taught in all major Indian forensic medicine textbooks (Parikh, Reddy, Modi). ### Why Acid Phosphatase is the First-Line Screening Test | Investigation | Role | Timing | Notes | |---|---|---|---| | **Acid phosphatase test** | **Initial presumptive screening** | 0–48 hours | Rapid, sensitive, standard first step | | **Wet mount microscopy** | Confirmatory (morphology) | 0–8 hrs (motile); 0–48 hrs (non-motile) | Performed after AP positive | | **DNA profiling** | Perpetrator identification | 0–72+ hours | Definitive but time-consuming | | **Blood grouping** | Population narrowing | 0–72+ hours | Rarely used in modern practice | ### Rationale for Acid Phosphatase as Initial Screen **High-Yield:** Acid phosphatase is present in seminal fluid at concentrations **500× higher** than in vaginal secretions. A positive AP test (colour change with sodium α-naphthyl phosphate + Fast Blue B dye) within 30 seconds is highly indicative of seminal fluid. It is: - **Rapid** (results in minutes) - **Sensitive** (detects semen even in azoospermic males) - **Cost-effective** and widely available in medico-legal settings **Clinical Pearl (Parikh's Textbook of Medical Jurisprudence & Toxicology):** The acid phosphatase test is described as the **standard presumptive/screening test** for seminal fluid. Wet mount microscopy is complementary and confirmatory, but AP is the *initial* biochemical screen. Importantly, AP remains positive even when spermatozoa are absent (e.g., vasectomised perpetrator), making it more broadly applicable as a screening tool. ### Why Wet Mount is NOT the Best Answer Here While wet mount microscopy is valuable within the 6–8 hour window for detecting motile spermatozoa, it: - Requires a skilled microscopist - Is negative in azoospermic males (~15% of males) - Is a **confirmatory** step, not the initial *screening* test per standard forensic protocols The stem asks for the "most appropriate **initial screening** investigation" — this aligns with acid phosphatase, not wet mount. ### Sequential Investigation Algorithm 1. **Acid phosphatase test** → Initial presumptive screen for seminal fluid 2. **Wet mount microscopy** → Confirm spermatozoa (motile/non-motile) 3. **DNA profiling** → Perpetrator identification 4. **Blood grouping** → Supplementary (rarely used) **Mnemonic:** **AWDB** — Acid phosphatase first, Wet mount second, DNA third, Blood grouping last *[Reference: Parikh's Textbook of Medical Jurisprudence, Forensic Medicine & Toxicology, 7th ed., Ch. 26; Reddy's The Essentials of Forensic Medicine & Toxicology, 34th ed.]*