18.3 Transfusion-Transmitted Diseases

18.3 Transfusion-Transmitted Diseases

Quick Answer: Residual risk is the infection that still sneaks through after history and licensed tests — mainly the window period. NAT made HIV and HCV residual risk extremely low; do not invent a 1-in-X decimal unless a named current source is in front of you. Current U.S. allogeneic testing is HIV Ab+NAT, HBV (HBsAg, anti-HBc, NAT), HCV Ab+NAT, HTLV-I/II, syphilis, and WNV NAT on each donation; T. cruzi one-time; Babesia NAT in endemic regions or pathogen reduction. CMV is a special-population label. Zika NAT is no longer routine. Malaria is travel deferral. vCJD is a theoretical residual. Lookback finds prior recipients. Pathogen reduction addresses bacteria and many parasites in platelets/plasma; it does not retire the viral NAT panel.

This is V.C.3. Chapter 3.1 already taught the release panel as a manufacturing gate. Chapter 2.1 already taught donor questions, including the shortened malaria travel deferral and the removal of most geographic vCJD deferrals. This section is transfusion practice: what still gets through, whom you protect with extra products, and what you do when a repeat donor later tests reactive.

Residual risk and the window period

No tested inventory is zero-risk. Residual risk is the chance a unit is infectious after donor qualification and licensed screening. The largest named piece for viruses we already test is the window period — the days between infection and the first detectable marker. Antibody tests have long windows. NAT detects viral nucleic acid days to weeks earlier, so the remaining infectious window for HIV and HCV shrank into a very small interval. That is why modern teaching says residual HIV and HCV transfusion risk is extremely low. HBV residual risk remains higher than HIV/HCV even with NAT, because of low-level occult infection and a longer residual window. If an item asks for a specific “1 in …” fraction, it must cite a named current source (FDA, AABB, or a dated national hemovigilance report). This guide will not invent one.

Other residual-risk sources: a testing error, an untested agent (malaria in U.S. allogeneic blood is controlled by history, not a routine licensed NAT), and an agent whose test is one-time or regional (T. cruzi, Babesia). Bacterial contamination of platelets is still a quantitative TTD problem compared with HIV; that is why 18.2 and Chapter 4.2 treat bacterial-risk control as part of the product.

Current U.S. allogeneic testing (stay aligned with 3.1)

As of the June 2026 eligibility frame, each allogeneic donation is tested with licensed donor-screening assays for:

Zika had nationwide NAT after 2016. In May 2021 FDA determined it no longer met the incidence/prevalence bar for an RTTI and withdrew the testing guidance. A 2026 stem that still demands Zika NAT on every U.S. allogeneic unit is using a retired rule, unless it is written as a history item.

Babesia is current. It is an intraerythrocytic parasite that survives 1–6 °C storage, clusters in the Northeast and Upper Midwest, and causes fever, hemolysis, and severe disease in asplenic or immunocompromised recipients. FDA guidance recommends year-round licensed Babesia NAT or an approved pathogen-reduction device for collections in Connecticut, Delaware, Maine, Maryland, Massachusetts, Minnesota, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, Vermont, Virginia, Wisconsin, and Washington, D.C. Outside those regions, ask the history of babesiosis question; do not pretend refrigeration sterilizes the unit.

T. cruzi (Chagas) is a one-time antibody screen. A nonreactive first test covers later donations from that donor under the FDA-accepted alternative. A reactive result interdicts the current unit and defers the donor. It is not a each-donation NAT.

Platelets still need a bacterial-risk strategy (pathogen reduction, LVDS, or primary-plus-secondary testing). That is TTD prevention, not an optional extra.

CMV, parasites, prions, and what you do not routinely test

CMV is not on the universal allogeneic panel. Most adults are already seropositive. The recipients who need a CMV-safe product are intrauterine transfusions, low-birth-weight neonates, and selected CMV-seronegative immunocompromised patients (including some HPC recipients). CMV-safe means prestorage leukoreduced (residual WBC < 5 × 10^6) or CMV-seronegative. Chapter 4.3 already equated leukoreduction with CMV-safe for exam purposes. Ordering both LR and CMV-seronegative is sometimes done for intrauterine work; it is not required for every red-cell issue.

Malaria has no routine U.S. donor laboratory test. Risk is managed by travel and residence deferral (Chapter 2.1 has the current shortened interval). A donor who had clinical malaria is deferred on a longer clock. Do not list malaria NAT next to HIV NAT on a 2026 U.S. panel item.

Other parasites to name: Babesia (above), T. cruzi (above), and, rarely, babesiosis as the clinical disease the recipient actually gets. Tick history in the recipient after a New England red-cell unit is a Babesia lookback trigger, not a Zika story.

Prions / vCJD remain a theoretical residual risk. There is no licensed donor screening test. FDA removed most geographic vCJD deferrals; do not resurrect a lifetime United Kingdom deferral as if it were still 2010 policy. Leukoreduction is not accepted on the exam as a proven vCJD-prevention strategy that lets you ignore residual theoretical risk.

Lookback and pathogen reduction

Lookback starts when a donor later tests reactive or confirmed-positive, or is implicated in a TTD. The collecting facility quarantines remaining in-date components and notifies consignees of prior collections inside the marker-specific window. HIV lookback is 21 CFR 610.46; HCV lookback is 21 CFR 610.47. Analogous retrieval applies to other markers under FDA guidance and AABB Standards. The transfusion service notifies the recipient’s physician so the recipient can be offered testing and counseling. Destroying only today’s reactive bag and hoping yesterday’s units were already transfused is not compliance.

Worked scenario. A repeat donor is HIV NAT reactive on Monday. Tuesday’s in-date plasma from last month is still in the hospital freezer. Quarantine it. Notify the consignee of earlier donations in the HIV lookback window. Offer recipient testing. Do not wait for a Western blot before freezing movement of in-date co-components.

Pathogen reduction in current U.S. practice is amotosalen plus UVA (INTERCEPT) for platelets and plasma. It is an accepted bacterial-risk strategy, inactivates many viruses and parasites (and is an alternative to Babesia NAT where the device is approved), and contributes to lymphocyte inactivation. It does not replace HIV/HBV/HCV NAT on the collection. It is not a licensed routine red-cell process that lets you skip the red-cell viral panel. Solvent-detergent pooled plasma is a manufacturer-specific pathogen-reduced plasma with its own circular. Fractionated derivatives (albumin, IVIG, RhIG) are pathogen-inactivated during industrial pooling — Chapter 4.4 — and are not a reason to call hospital FFP “sterile.”

Exam traps. Do not still require Zika NAT. Do not make Babesia NAT a nationwide every-donation rule. Do not put CMV on the universal panel. Do not invent a residual-risk decimal. Do not say NAT made lookback obsolete. Do not call pathogen-reduced platelets a reason to skip HIV NAT. Do not treat malaria as a laboratory release test.