Author: Yang Ruifeng
Unit: Peking University People's Hospital
Human immunodeficiency virus (HIV) infection and AIDS are mainly transmitted through sexual transmission, which is a great threat to health, and there is a lack of radical treatment. In clinical or disease control, we will always encounter high-risk behaviors after suspected of being infected, seeking testing again and again, holding multiple "negative" report sheets but still can not get rid of anxiety of the subjects; in the online world, the posts that consult the interpretation of HIV-negative report sheets can also account for more than half of the content of Tieba Bar, and paid consultations and paid onlookers under Weibo are also everywhere.
In fact, after the diagnosis of HIV after high-risk behavior, there is only one core concept, that is, the "window period", fully understand this concept, all doubts can be solved. Moreover, the window period issue has been fully studied, and it is not necessary to continue to worry about it, or to pay for some advice that may not be reliable. This article aims to popularize diagnostic common sense while also trying to keep some people's pocketbooks.
First, what is the window period, and how does its concept change?
Window period (window period) is one of many imported words, window has the meaning of "gap", can refer to good timing, such as rockets need to choose the wind and sunny window launch, can also refer to bad things, such as a person after falling out of love sad, delay in starting a new relationship, then, this period is his feelings of the window, this sad color of the window applied to the field of test diagnosis, can be used to refer to the microbial invasion of the early human body, It is not possible to diagnose the existence of gaps using existing testing methods, but it is easier to understand the translation of "empty window period".
The window period in the traditional sense refers to the antibody window period. This concept is still used in the World Health Organization and the European Guidelines for the Diagnosis of HIV (2014). Over the years, with the development of HIV testing techniques, the definition of window period has gradually expanded to the window period of nucleic acids (HIV RNA) or serological markers (IgM, IgG and p24). From the 2008 edition to the 2019 edition of the Chinese HIV/AIDS diagnosis and health industry standards, the expansion of this concept is reflected, see the figure below.
Second, how is the window period formed, and how does it affect the diagnosis?
After exposure and inoculation of HIV, the virus invades the cell and multiplies intracellularly, but does not destroy the cell, there is no virus outside the cell, and no virus can be found by any method, this period is called the eclipse period (which can be translated into the covert period); after that, the HIV destroys the cell membrane, goes out of the cell and looks for a new habitat, enters the ramp-up phase (the ascent phase, which can be understood as the beginning of the viremia phase), and the infected person's peripheral blood begins to appear traces of HIV RNA. However, it is still not detectable by HIV RNA reagents. After that, the virus grows exponentially, becoming positive for nucleic acid when the concentration exceeds the minimum detection limit for RNA reagents. From exposure to the virus to positive nucleic acid testing, this period is the window period for nucleic acid testing, including the concealment period and the uplift period.
Viral antigens such as outer membrane proteins pg120 and gp41, core proteins (p24), etc., appear with RNA at the same time. The outer membrane protein of HIV is highly variable and difficult to detect stably, and we generally detect p24. Since immunological testing cannot exponentially amplify target genes by polymerase chain reaction (PCR) like nucleic acids, the sensitivity of antigens is 1 to 2 orders of magnitude lower than nucleic acid detection, so the p24 window period is also later than that of RNA.
After the antigen appears, the immune system begins to produce corresponding antibodies, such as pg120, gp41 and p24 antibodies. The window period of IgM is shorter than that of IgG.
It can be seen that the window period of different HIV markers is different, and at the same time, the window period is also related to the sensitivity of the reagent, and the higher the sensitivity, the shorter the window period. With the sensitivity of current molecular or serological techniques, the RNA, p24, IgM, and IgG window periods are ~1.5 weeks, ~2 weeks, ~3 weeks, and ~4 weeks, respectively, as shown in the figure below.
Third, why are the window periods of different "generations" of immunological reagents different?
From the 1980s to the present, HIV immunological diagnostic reagents have been iteratively developed (from the 1st to the 5th generation, see the figure below), the higher the generation, the more recent the technology, the more complete the detected viral markers, so the shorter the window period.
Where do the samples used to study the window period of HIV infection come from?
Window studies are part of the study of the natural history of HIV infection in humans. There are research teams or reagent companies that detect HIV markers by prospectively collecting serial blood samples from subjects who may be infected with HIV. If certain subjects do become infected with HIV during the course of the project, blood samples before and after infection are collected for research. This kind of forward-looking project requires extensive screening and tracking of subjects, which consumes a lot of manpower and material resources, so commercial "seroconversion panels" made of a series of serums from different HIV-infected people are extremely expensive. On this basis, combined with various statistical methods, the window period can be calculated.
Why is there a significant difference in the length of the recommended window period in some countries?
According to the Continental Health Industry Standard (2019), the window periods for HIV antibodies, antigens and nucleic acids are about 3 weeks, 2 weeks and 1 week after infection, respectively, which is roughly similar to the US CDC HIV Laboratory Test Recommendation (2014). However, in the UK AIDS Association's HIV testing guidelines (2020), the window period is much longer:
l 4th generation laboratory reagents (non-POCT): 45 days
l 3rd generation laboratory reagents (non-POTT): 60 days
All POCT reagents (laboratory or home self-test): 90 days
This apparent inconsistency stems from different algorithms. The UK guidelines recommend that the window length should be calculated based on the 99th percentile, which can be understood as the window length of the 99th percentile, or as the length of time it takes for 99% of infected people to turn positive, in any case, such a calculation will lead to a significantly longer window period (see figure below), for example, the 3rd generation reagent is about 50 days, while the 4th generation reagent is about 45 days. The advantage of a longer window period is that the "false negative" of the result can be completely excluded (P
But the other side of the coin is that the frequency of HIV-infected people is not normally distributed over time, but in a left-biased distribution, that is, the HIV markers of infected people are concentrated in a short period of time in the early stage of exposure, and only a small number of infected people turn to yang very late, forming a long time tail. The window period given by countries such as China and the United States is based on the median estimate, which is much shorter; but considering that the concentration of infection occurs near the median window period, even if the test is shortly after the window period, a report with a predictable value can be obtained - positive results can be found in time for very early infection, negative results can also exclude infection to a certain extent, and the subject does not have to wait too long. However, the diagnostic value of a negative result in the median window period is ultimately less than that of the 99th percentile window period – which is also the anxiety of some people with "AIDS phobia".
6. What are the individual factors affected by the window period?
The window period is a statistical result based on population data, and for individuals, it is also affected by many factors, such as:
1. Exposure to viral volume: The more virus particles inoculated, the shorter the window period.
2. Exposure routes: sexual, occupational exposure, intravenous drug use, blood transfusion and other different transmission routes affect the window period.
3. Gene subtype of the virus strain: Europe and the United States are mainly B subtypes, window period data is also mainly based on European and American data, and the continental subtype is very different from Europe and the United States, for example, the mainstream CRF01_AE subtype in the mainland, mainly based on same-sex transmission, has stronger pathogenicity, and the number of years of progression to AIDS is shorter, for this reason, the latest AIDS diagnosis and treatment guidelines in the mainland will shorten the asymptomatic period of mainland patients from the previous 6 to 8 years to 4 to 8 years. However, there are not many reports of the window period of this subtype of virus.
4. The immune system responds to HIV: the stronger the specific immunity, the shorter the antibody window period.
5. Early treatment, or prophylactic therapy including pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP): antiviral drugs inhibit HIV RNA replication, delaying the production of antibodies, which may prolong the window period.
Back to the top question: What scientific and effective advice should be provided to individuals seeking HIV testing after high-risk behaviors?
In summary, a negative result after the median window period initially excludes infection, and a negative result after the 99th percentile window completely excludes infection. The author summarizes the recommended detection time points and significance of different reagents, see the figure below.
(1) If you go to a medical laboratory for testing:
HIV RNA can be detected best, its window period is the shortest, and the median and 99 percentile times of HIV RNA in infected patients are 11.5 days and 33 days, respectively. However, it is lowly developed, expensive, and the performance of the reagent needs further evaluation - on the one hand, the risk of false negative caused by the mutation of the viral nucleic acid sequence cannot be completely ruled out; on the other hand, the false positive result caused by PCR pollution will make people "false alarm". Due to the early stages of HIV infection, the vast majority of infected people tend to have a very high viral load, and "low-value positive" RNA results are more likely to be false positives.
3rd or 4th generation immunological reagents: reagents are mature, stable in performance, widely developed, and inexpensive. The window period of the 4th generation reagent is about 1 week shorter than that of the 3rd generation, although the subject generally does not know the first generation of the reagent in advance, and some laboratories do not even indicate the reagent information, in which case a negative result of 2 months can exclude infection.
(2) If you choose self-test:
Home self-test reagents are mainly immediate reagents (POCT, that is, rapid reagents), the sample is fingertip blood, saliva or urine, the sensitivity is not as good as the laboratory reagents based on serum or plasma, and there is currently no sufficient research data to support the calculation window period, for the sake of safety, a longer window period should be assigned, and the Self-test reagent window period recommended by the United Kingdom and the United States is 90 days.
(3) If there are pre-exposure prophylaxis (PrEP) or post-exposure prophylaxis (PEP) measures:
The use of antiviral drugs complicates the diagnosis of HIV infection and may require more frequent and longer surveillance. Fortunately, cases of HIV infection despite standardized prophylactic treatment are very rare.
PrEP: HIV testing is recommended every 3 months.
PEP: HIV testing is recommended at 3 to 6 weeks, 3 months, and 6 months.
【Reference】
1. Sex Transm Dis. 2017,44:739-46.
2. Int J STD AIDS. 2015,26:215-24.
3. Clin Infect Dis. 2017,64:53-9.
4. Journal of Clinical Laboratory Research,2021,39:809-12.
5. https://stacks.cdc.gov/view/cdc/23447.
6. Clin Vaccine Immunol. 2016, 23:249-53.
7.https://www.cdc.gov/hiv/pdf/testing/cdc-hiv-faq-nonclinical-hiv-testing.pdf
8. HIV Med. 2020, 21 Suppl 6:1-26.
Editor: Ren Mileage Reviewer: Xiao Ran