A probabilistic method for the estimation of residual risk in donated blood

A methodology for deriving the sensitivity of pooled testing, based on viral load progression and pooling dilution

BACKGROUND

Pooled testing, in which biological specimens from multiple subjects are combined into a testing pool and tested via a single test, is a common testing method for both surveillance and screening activities. The sensitivity of pooled testing for various pool sizes is an essential input for surveillance and screening optimization, including testing pool design. However, clinical data on test sensitivity values for different pool sizes are limited, and do not provide a functional relationship between test sensitivity and pool size. We develop a novel methodology to accurately compute the sensitivity of pooled testing, while accounting for viral load progression and pooling dilution. We demonstrate our methodology on the nucleic acid amplification testing (NAT) technology for the human immunodeficiency virus (HIV).

METHODS

Our methodology integrates mathematical models of viral load progression and pooling dilution to derive test sensitivity values for various pool sizes. This methodology derives the conditional test sensitivity, conditioned on the number of infected specimens in a pool, and uses the law of total probability, along with higher dimensional integrals, to derive pooled test sensitivity values. We also develop a highly accurate and easy-to-compute approximation function for pooled test sensitivity of the HIV ULTRIO Plus NAT Assay. We calibrate model parameters using published efficacy data for the HIV ULTRIO Plus NAT Assay, and clinical data on viral RNA load progression in HIV-infected patients, and use this methodology to derive and validate the sensitivity of the HIV ULTRIO Plus Assay for various pool sizes.

RESULTS

We demonstrate the value of this methodology through optimal testing pool design for HIV prevalence estimation in Sub-Saharan Africa. This case study indicates that the optimal testing pool design is highly efficient, and outperforms a benchmark pool design.

CONCLUSIONS

The proposed methodology accounts for both viral load progression and pooling dilution, and is computationally tractable. We calibrate this model for the HIV ULTRIO Plus NAT Assay, show that it provides highly accurate sensitivity estimates for various pool sizes, and, thus, yields efficient testing pool design for HIV prevalence estimation. Our model is generic, and can be calibrated for other infections.

Emerging Infectious Diseases and Blood Safety: Modeling the Transfusion-Transmission Risk

While the transfusion-transmission (TT) risk associated with the major transfusion-relevant viruses such as HIV is now very low, during the last 20 years there has been a growing awareness of the threat to blood safety from emerging infectious diseases, a number of which are known to be, or are potentially, transfusion transmissible. Two published models for estimating the transfusion-transmission risk from EIDs, referred to as the Biggerstaff-Petersen model and the European Upfront Risk Assessment Tool (EUFRAT), respectively, have been applied to several EIDs in outbreak situations. We describe and compare the methodological principles of both models, highlighting their similarities and differences. We also discuss the appropriateness of comparing results from the two models. Quantitating the TT risk of EIDs can inform decisions about risk mitigation strategies and their cost-effectiveness. Finally, we present a qualitative risk assessment for Zika virus (ZIKV), an EID agent that has caused several outbreaks since 2007. In the latest and largest ever outbreak, several probable cases of transfusion-transmission ZIKV have been reported, indicating that it is transfusion-transmissible and therefore a risk to blood safety. We discuss why quantitative modeling the TT risk of ZIKV is currently problematic.

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During the last 20 years there has been a growing awareness of the threat to blood safety from emerging infectious diseases (EIDs), a number of which are known to be, or are potentially, transfusion-transmissible.

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The transfusion-transmission risk of EID agents can be estimated by risk modeling which can form an important part of risk assessments and inform decisions regarding risk mitigation strategies.

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We describe and compare the methodological principles of two published risk models for estimating the transfusion transmission risk of EIDs.

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We use Zika virus as a case study to demonstrate that reliable risk modeling for EID agents can be problematic due to the uncertainty of the input parameters.

Selection strategies for newly registered blood donors in European countries

BACKGROUND

Two selection strategies for newly-registered blood donors are available: a single-visit selection called the standard selection procedure (SSP), and a two-stage selection named predonation and donation screening (PDS). This study reviews the selection strategies for newly-registered donors currently applied in European countries.

MATERIAL AND METHODS

We collected data on donor selection procedures, blood donation, laboratory screening and HIV, HCV and HBV positive donors/donations from 2010 to 2013 in 30 European countries by using questionnaires. We grouped the countries according to the applied selection strategy, and for each country, we calculated the 4-year prevalence of confirmed positive results indicating the presence of overall and recent HIV, HCV and HBV infections among first-time and repeat donations and among newly-registered donors.

RESULTS

Most of the 24 countries (80%) apply the SSP strategy for selection of newly-registered donors. Twenty-two countries (73.3%) employ a nucleic acid amplification testing in addition to the mandatory serological screening. The survey confirms a higher overall prevalence of HIV, HCV and HBV infections among first-time donations and newly-registered donors than among repeat donations. In contrast, the prevalence of recently acquired HIV and HCV infections was lower among first-time donations and newly-registered donors than among repeat donations, but higher for recent HBV infections (6.7/10⁵ vs 2.6/10⁵ in the SSP setting and 4.3/10⁵ vs 0.5/10⁵ in one country using PDS). The relatively low numbers of infected donors selected by PDS impeded accurate assessment of the prevalence of recent infections in first-time donations.

DISCUSSION

The data from European countries provide inconclusive evidence that applying PDS reduces the risk of donations being made in the diagnostic window of first-time donors. The impact of PDS on the risk of window-period donations and blood donor management needs further investigation.

Residual risk and waste in donated blood with pooled nucleic acid testing

An accurate estimation of the residual risk of transfusion-transmittable infections (TTIs), which includes the human immunodeficiency virus (HIV), hepatitis B and C viruses (HBV, HCV), among others, is essential, as it provides the basis for blood screening assay selection. While the highly sensitive nucleic acid testing (NAT) technology has recently become available, it is highly costly. As a result, in most countries, including the United States, the current practice for human immunodeficiency virus, hepatitis B virus, hepatitis C virus screening in donated blood is to use pooled NAT. Pooling substantially reduces the number of tests required, especially for TTIs with low prevalence rates. However, pooling also reduces the test's sensitivity, because the viral load of an infected sample might be diluted by the other samples in the pool to the point that it is not detectable by NAT, leading to potential TTIs. Infection-free blood may also be falsely discarded, resulting in wasted blood. We derive expressions for the residual risk, expected number of tests, and expected amount of blood wasted for various two-stage pooled testing schemes, including Dorfman-type and array-based testing, considering infection progression, infectivity of the blood unit, and imperfect tests under the dilution effect and measurement errors. We then calibrate our model using published data and perform a case study. Our study offers key insights on how pooled NAT, used within different testing schemes, contributes to the safety and cost of blood. Copyright © 2016 John Wiley & Sons, Ltd.