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PA21.1-6 | Transfusion-Transmitted Infections — Part 1
CLINICAL SCENARIO
In 2019, a 28-year-old patient received two units of packed red cells after elective surgery in a district hospital. Three months later, she tested positive for HIV. The investigation traced the infection to a donor who had been in the immunological window period — the ELISA for HIV was negative, yet the donor was already infectious. Her case triggered a look-back exercise covering 47 previous recipients from the same donor. This is not a hypothetical: the NACO haemovigilance registry documents dozens of such events annually. Every blood unit you order carries a residual risk — your job is to know that risk, minimise it, and counsel patients honestly.
WHY THIS MATTERS
As a Year-2 student, you are building the foundational knowledge to become a safe prescriber of blood products. Transfusion medicine sits at the intersection of microbiology, immunology, and clinical practice. The infections covered in this module — HIV, hepatitis B, hepatitis C, syphilis, malaria — are the five legally mandated screening targets under Indian law. Missing any one of them is a medicolegal event. Beyond the mandatory tests, you will encounter patients who are immunocompromised or neonates, where even CMV-negative status of the donor matters. Bacterial contamination of platelets is the commonest infectious cause of transfusion-related death worldwide and in India — a fact most students do not know until it is examined.
RECALL
Before proceeding, activate what you already know:
- From Microbiology (Year 1): What is the difference between a screening test and a confirmatory test? Give an example for HIV.
- From Physiology (Year 1): How does the immune system respond in the first 2–3 weeks after viral exposure before antibodies are detectable?
- From Biochemistry (Year 1): What does PCR detect — DNA/RNA, or antibody/antigen?
- Clinical reasoning: If a test is 99.9% specific and India screens 12 million units annually, how many false positives would you expect each year?
Jot brief answers before moving on — this module will extend all four concepts.
Why TTIs Matter: The Iatrogenic Risk
Why TTIs Matter: The Iatrogenic Risk
A transfusion-transmitted infection (TTI) is any infection acquired by a recipient through the transfusion of blood or a blood component. The defining characteristic is that it is iatrogenic — caused by medical intervention — and therefore preventable through systematic blood safety measures.
The public health significance in India is substantial:
- Over 12 million blood units are collected annually.
- Voluntary non-remunerated donation rates are rising but replacement/paid donation persists in some states.
- Infrastructure gaps mean NAT testing is not yet universal across all blood banks.
TTIs impose a dual burden: the infection itself (potentially HIV, chronic hepatitis, or fatal sepsis) and the erosion of public trust in the blood supply. The haemovigilance system — systematic monitoring and reporting of adverse transfusion events — exists precisely to quantify and reduce this burden. India's national haemovigilance programme (HvPI) has operated since 2012 under the Indian Pharmacopoeia Commission.
Mandatory NACO Screening Tests: The Five Pillars
The National AIDS Control Organisation (NACO) and National Blood Transfusion Council (NBTC) mandate testing of every donated unit for five infections under the Drugs and Cosmetics Act. No unit may be issued without a negative result on all five.
IMPORTANT: The table below summarises the five mandatory tests. Learn it as a clinical fact — examiners ask for the organism, the test, and the window period together.
NACO Mandatory Screening Tests for Blood Donation in India
HIV 1 and 2: Detected by fourth-generation ELISA, which simultaneously detects p24 antigen (appears at 2 weeks) and anti-HIV antibody (appears at 3–4 weeks). Combined antigen-antibody testing significantly reduced the window period compared to older antibody-only assays. Confirmatory testing (Western blot or line immunoassay) is mandatory for reactive units before discarding.
Hepatitis B (HBsAg): The hepatitis B surface antigen (HBsAg) is the principal marker of active HBV infection. It becomes detectable 1–10 weeks after exposure. The window period is longer than HIV — donated units from donors in the HBsAg-negative window can still transmit HBV, which is why anti-HBc (core antibody) testing is recommended in high-prevalence settings (though not yet universally mandated in India).
Hepatitis C (anti-HCV): The screening test detects antibody to HCV proteins. Because HCV causes chronic infection in ~70% of cases and has no vaccine, the transfusion risk is particularly significant. Current 3rd-generation anti-HCV ELISAs have narrowed the window period substantially.
Syphilis (VDRL/TPHA): Treponema pallidum is the causative organism. Treponema pallidum does not survive beyond 72–96 hours in refrigerated blood, so the risk from stored red cells is low — but fresh components (platelets, FFP) and direct transfusion remain a real risk. VDRL is the non-treponemal (reagin) test; TPHA (Treponema pallidum haemagglutination assay) is more specific.
Malaria: India has significant malaria endemicity, making this mandatory. Screening uses thick blood smear microscopy and/or rapid malarial antigen tests. Plasmodium falciparum and P. vivax are the predominant species. The parasites survive in refrigerated red cells but not in FFP or cryoprecipitate (freeze-kill effect).
SELF-CHECK
Which of the following is NOT included in the five mandatory NACO screening tests for blood donation in India?
A. Anti-HCV ELISA
B. HBsAg ELISA
C. Anti-HTLV I/II
D. VDRL/TPHA for syphilis
Reveal Answer
Answer: C. Anti-HTLV I/II
The five mandatory NACO tests are HIV (ELISA), HBsAg (Hepatitis B), anti-HCV (Hepatitis C), VDRL/TPHA (Syphilis), and malarial antigen/smear. Anti-HTLV I/II is NOT currently mandated in India — HTLV screening is recommended in some high-risk populations in other countries but is not part of the Indian mandatory panel.
The Window Period and NAT Testing
Window Period and NAT Testing in Blood Safety
The window period is the interval between infection of a donor and the point at which a standard screening test turns positive. During this time, the donor is infectious but the unit tests negative — the most dangerous gap in blood safety.
Why does it exist? Serological tests detect antibody (or antigen in combined assays). Antibody production (seroconversion) takes time — the immune system must first encounter the pathogen, mount an adaptive response, and produce sufficient immunoglobulin to be detected. During this lag, viral load may already be high.
IMPORTANT: The diagram below illustrates the window period timeline for HIV and HCV, showing viral load, p24 antigen, and antibody curves against time after exposure, with the NAT-detectable window vs the serology-detectable window labelled.
Window Period Dynamics in HIV and HCV Infections
Nucleic acid testing (NAT) directly detects viral RNA or DNA — it does not depend on the host immune response. Because viral nucleic acid is present earlier than detectable antibody or antigen, NAT substantially shortens the window period:
| Agent | Serology window | NAT window |
|---|---|---|
| HIV | 18–45 days | 9–11 days |
| HCV | 54–70 days | 7–10 days |
| HBV | 38–68 days | 15–34 days |
NAT testing is now recommended for all licensed blood banks in India under the 2022 NBTC standards and is mandatory for government banks in many states. However, residual risk — the irreducible risk even with NAT — is not zero: it depends on yield (viral copies per mL) at the time of donation and assay sensitivity. For HIV, the current residual risk with NAT is estimated at approximately 1 in 1–2 million donations in high-income countries.
CLINICAL PEARL
The window period is the Achilles heel of blood safety. A single-donation NAT pool (individual NAT, or ID-NAT) is more sensitive than mini-pool NAT (pooling 6–24 samples and testing together) because dilution in mini-pools raises the detection threshold. India currently uses a mix of ID-NAT and mini-pool NAT depending on blood bank capacity. When counselling a patient asking 'Is this blood completely safe?', the honest answer is: 'The risk is extremely low and systematically minimised — but not zero.'