The Use of Mathematical Models of Chlamydia Transmission to Address Public Health Policy Questions: A Systematic Review

Public willingness to receive chlamydia, gonorrhea, syphilis, and trichomoniasis vaccines: a scoping review

BACKGROUND

Sexually transmitted infections (STIs) like chlamydia, gonorrhea, syphilis, and trichomoniasis contribute significantly to global morbidity and mortality. Researchers are pursuing vaccines for these STIs, and a clinical trial is currently underway for a chlamydia vaccine. However, there is little research available on individuals' willingness to receive chlamydia, gonorrhea, syphilis, and trichomoniasis vaccines. The purpose of this analysis was to map the existing literature we have on individuals' willingness to receive these bacterial/parasitic STI vaccines and understand what information on vaccine acceptability is still needed.

METHODS

We searched seven databases for literature on STI vaccine acceptability, then conducted title/abstract and full-text reviews to assess eligibility. All reviews and abstractions were conducted blindly by two reviewers, with discrepancies settled by discussion or the input of a third reviewer.

RESULTS

Eight of the original 2,259 texts of interest met inclusion criteria. After data abstraction, we found that gonorrhea was the most commonly examined, followed by chlamydia and syphilis. Trichomoniasis vaccine acceptability was not reported. Most texts reported high acceptability, but there did not appear to be data describing how vaccine characteristics affect acceptability. Similarly, while the literature covers a variety of populations, most of the study populations were based out of the United States or Canada and were patrons of healthcare facilities or participants from a larger health intervention study. Therefore, more information is needed on populations outside North America, and on groups with lower healthcare access and utilization.

CONCLUSION

As the incidence of bacterial and parasitic STIs increase, and as we grow nearer vaccines for these illnesses, understanding how likely the public is to accept and receive these vaccines is crucial to their success. While the existing literature describes STI vaccine acceptability in a variety of populations, their overall number is small. More research into STI vaccine acceptability outside of North America, and especially examining how factors like number of doses, timing, and cost influence vaccine acceptability is needed to ensure effective future vaccine rollouts.

Costs, Health Benefits, and Cost-Effectiveness of Chlamydia Screening and Partner Notification in the United States, 2000-2019: A Mathematical Modeling Analysis

BACKGROUND

Chlamydia remains a significant public health problem that contributes to adverse reproductive health outcomes. In the United States, sexually active women 24 years and younger are recommended to receive annual screening for chlamydia. In this study, we evaluated the impact of estimated current levels of screening and partner notification (PN), and the impact of screening based on guidelines on chlamydia associated sequelae, quality adjusted life years (QALYs) lost and costs.

METHODS

We conducted a cost-effectiveness analysis of chlamydia screening, using a published calibrated pair formation transmission model that estimated trends in chlamydia screening coverage in the United States from 2000 to 2015 consistent with epidemiological data. We used probability trees to translate chlamydial infection outcomes into estimated numbers of chlamydia-associated sequelae, QALYs lost, and health care services costs (in 2020 US dollars). We evaluated the costs and population health benefits of screening and PN in the United States for 2000 to 2015, as compared with no screening and no PN. We also estimated the additional benefits that could be achieved by increasing screening coverage to the levels indicated by the policy recommendations for 2016 to 2019, compared with screening coverage achieved by 2015.

RESULTS

Screening and PN from 2000 to 2015 were estimated to have averted 1.3 million (95% uncertainty interval [UI] 490,000-2.3 million) cases of pelvic inflammatory disease, 430,000 (95% UI, 160,000-760,000) cases of chronic pelvic pain, 300,000 (95% UI, 104,000-570,000) cases of tubal factor infertility, and 140,000 (95% UI, 47,000-260,000) cases of ectopic pregnancy in women. We estimated that chlamydia screening and PN cost $9700 per QALY gained compared with no screening and no PN. We estimated the full realization of chlamydia screening guidelines for 2016 to 2019 to cost $30,000 per QALY gained, compared with a scenario in which chlamydia screening coverage was maintained at 2015 levels.

DISCUSSION

Chlamydia screening and PN as implemented in the United States from 2000 through 2015 has substantially improved population health and provided good value for money when considering associated health care services costs. Further population health gains are attainable by increasing screening further, at reasonable cost per QALY gained.

Mathematical model for transmission of Chlamydia due to sexual activity and unhygienic environment

Aim: Sexually transmitted diseases (STDs) need to be studied systematically to better understand their global spread. Transmission of Chlamydia trachomatis is a severe public health issue, with roughly 90 million new cases per year. Globally, Chlamydia trachomatis is the most frequent bacterial cause of STDs. Methods: To better understand the dynamics and transmission of Chlamydia, the susceptible-exposed-infected-recovered-susceptible (SEIRS) model was constructed. Using a system of nonlinear ordinary differential equations, a basic reproduction number has been calculated at an equilibrium point, and the system is locally and globally asymptotically stable at both disease-free and endemic equilibrium points. Numerical simulations illustrate the behavior and flow of Chlamydia infections in different compartments. Results: Conclude from the proposed study that 25% of individuals have been exposed to Chlamydia, of which 20% of individuals get infections due to sexual activity and 55% of individuals get recovered. 20% of individuals have been exposed to Chlamydia, of which 37% of individuals have been infected due to an unhygienic environment. Of those, 43% of individuals recovered. Also, it has been found that people are more likely to get infections because of an unhygienic environment than sexually active people. The recovery rate is also much better for people who have been infected because of an unhygienic environment. Conclusions: Sexually transmitted infections can be reduced by up to 10%. While infection due to an unhygienic environment can be controlled up to a certain intensity. According to this research, public awareness campaigns and the improvement of personal hygiene will play a major role in reducing the spread of the epidemic in the future.

Intensified partner notification and repeat testing can improve the effectiveness of screening in reducing Chlamydia trachomatis prevalence: a mathematical modelling study

BACKGROUND

The Australian Chlamydia Control Effectiveness Pilot (ACCEPt) was a cluster randomised controlled trial designed to assess the effectiveness of annual chlamydia testing through general practice in Australia. The trial showed that testing rates increased among sexually active men and women aged 16-29 years, but after 3 years the estimated chlamydia prevalence did not differ between intervention and control communities. We developed a mathematical model to estimate the potential longer-term impact of chlamydia testing on prevalence in the general population.

METHODS

We developed an individual-based model to simulate the transmission of Chlamydia trachomatis in a heterosexual population, calibrated to ACCEPt data. A proportion of the modelled population were tested for chlamydia and treated annually at coverage achieved in the control and intervention arms of ACCEPt. We estimated the reduction in chlamydia prevalence achieved by increasing retesting and by treating the partners of infected individuals up to 9 years after introduction of the intervention.

RESULTS

Increasing the testing coverage in the general Australian heterosexual population to the level achieved in the ACCEPt intervention arm resulted in reduction in the population-level prevalence of chlamydia from 4.6% to 2.7% in those aged 16-29 years old after 10 years (a relative reduction of 41%). The prevalence reduces to 2.2% if the proportion retested within 4 months of treatment is doubled from the rate achieved in the ACCEPt intervention arm (a relative reduction of 52%), and to 1.9% if the partner treatment rate is increased from 30%, as assumed in the base case, to 50% (a relative reduction of 59%).

CONCLUSION

A reduction in C. trachomatis prevalence could be achieved if the level of testing as observed in the ACCEPt intervention arm can be maintained at a population level. More substantial reductions can be achieved with intensified case management comprising retesting of those treated and treatment of partners of infected individuals.

African based researchers' output on models for the transmission dynamics of infectious diseases and public health interventions: A scoping review

BACKGROUND

Applied epidemiological models are used in predicting future trends of diseases, for the basic understanding of disease and health dynamics, and to improve the measurement of health indicators. Mapping the research outputs of epidemiological modelling studies concerned with transmission dynamics of infectious diseases and public health interventions in Africa will help to identify the areas with substantial levels of research activities, areas with gaps, and research output trends.

METHODS

A scoping review of applied epidemiological models of infectious disease studies that involved first or last authors affiliated to African institutions was conducted. Eligible studies were those concerned with the transmission dynamics of infectious diseases and public health interventions. The review was consistent with the Preferred Reporting Items for Systematic Reviews and Meta-Analysis (PRISMA) extension for scoping reviews. Four electronic databases were searched for peer-reviewed publications up to the end of April 2020.

RESULTS

Of the 5927 publications identified, 181 met the inclusion criteria. The review identified 143 publications with first authors having an African institutional affiliation (AIA), while 81 had both first and last authors with an AIA. The publication authors were found to be predominantly affiliated with institutions based in South Africa and Kenya. Furthermore, human immunodeficiency virus, malaria, tuberculosis, and Ebola virus disease were found to be the most researched infectious diseases. There has been a gradual increase in research productivity across Africa especially in the last ten years, with several collaborative efforts spread both within and beyond Africa.

CONCLUSIONS

Research productivity in applied epidemiological modelling studies of infectious diseases may have increased, but there remains an under-representation of African researchers as leading authors. The study findings indicate a need for the development of research capacity through supporting existing institutions in Africa and promoting research funding that will address local health priorities.

Partnership dynamics in mathematical models and implications for representation of sexually transmitted infections: a review

Mathematical models of sexually transmitted disease (STI) are increasingly relied on to inform policy, practice, and resource allocation. Because STI transmission requires sexual contact between two or more people, a model's ability to represent the dynamics of sexual partnerships can influence the validity of findings. This ability is to a large extent constrained by the model type, as different modeling frameworks vary in their capability to capture patterns of sexual contact at individual, partnership, and network levels. In this paper, we classify models into three groups: compartmental, individual-based, and statistical network models. For each framework, we describe the basic model structure and discuss key aspects of sexual partnership dynamics: how and with whom partnerships are formed, partnership duration and dissolution, and temporal overlap in partnerships (concurrency). We illustrate the potential implications of accurately accounting for partnership dynamics, but these effects depend on characteristics of both the population and pathogen; the combined impact of these partnership and epidemiologic dynamics can be difficult to predict. While each of the reviewed model frameworks may be appropriate to inform certain research or policy questions, modelers and consumers of models should carefully consider the implications of sexual partnership dynamics for the questions under study.

Impact of screening on the prevalence and incidence of Mycoplasma genitalium and its macrolide resistance in men who have sex with men living in Australia: A mathematical model

BACKGROUND

Mycoplasma genitalium (MG) causes a sexually transmitted infection (STI) with a rising rate of antimicrobial resistance. Currently, guidelines do not recommend screening asymptomatic men who have sex with men (MSM). We developed a mathematical model of MG transmission to examine the impact of various screening strategies on the incidence and prevalence of MG among MSM attending a sexual health clinic.

METHODS

A compartmental mathematical model of MG transmission among MSM was constructed and calibrated using data from the Melbourne Sexual Health center, where resistance-guided therapy provides high treatment effectiveness (92-95%). The model stratified men by symptom status, sexual risk behaviours and whether or not they had MG with macrolide resistance. We simulated the impact on endemic steady-state MG prevalence and incidence of the following screening scenarios, namely screening: 1) no MSM; 2) only symptomatic MSM (the current recommendation); 3) all symptomatic and high-risk asymptomatic MSM; and 4) all MSM. Our base case analysis assumed a treatment effectiveness of 92-95% using resistance-guided therapy. We also examined the impact of treatment effectiveness (i.e. the proportion of detected MG that were cured) and screening coverage (i.e. testing rate) on MG prevalence.

FINDINGS

The model predicts that the overall endemic MG prevalence is 9.1% (95% CI: 7.9-10.0) in the current situation where screening is only offered to symptomatic MSM (base-case). This would increase to 11·4% (95% confidence intervals (CI): 10.2-13.7) if no MSM are offered screening, but would decrease to 7.3% (95% CI: 5.7-8.4) if all symptomatic and high-risk asymptomatic MSM were offered screening and 6.4% (95% CI: 4.7-7·7) if all MSM were offered screening. Increasing coverage of MSM screening strategies shows a similar effect on decreasing endemic MG incidence. When evaluating the simultaneous impact of treatment effectiveness and screening coverage, we found that offering screening to more MSM may reduce the overall prevalence but leads to a higher proportion of macrolide-resistant MG, particularly when using treatment regimens with lower effectiveness.

INTERPRETATION

Based on the available treatment options, offering screening for MG to other MSM (beyond the currently recommended group of symptomatic MSM) could slightly reduce the prevalence and incidence of MG. However, further increasing screening coverage must be weighed against the impact of lower treatment effectiveness (i.e. when not using resistance-guided therapy), increasing the selection of macrolide resistance, and other negative consequences related to AMR and management (e.g. unnecessary psychological morbidity from infections that do not need treatment).

Modelling the impact of tailored behavioural interventions on chlamydia transmission

Behavioural interventions tailored to psychological characteristics of an individual can effectively achieve risk-reducing behaviour. The impact of tailored interventions on population-level chlamydia prevalence is unknown. We aimed to assess the impact on overall chlamydia prevalence five years after the introduction of an intervention aimed at increasing self-efficacy, social norms, attitudes and intentions towards condom use (i.e., condom intervention), and an intervention aimed at increasing health goals and decreasing impulsiveness (i.e., impulsiveness intervention). A pair model, informed by longitudinal psychological and behavioural data of young heterosexuals visiting sexual health centers, with susceptible-infected-susceptible structure was developed. The intervention effect was defined as an increased proportion of each subgroup moving to the desired subgroup (i.e., lower risk subgroup). Interventions tailored to subgroup-specific characteristics, assuming differential intervention effects in each subgroup, more effectively reduced overall chlamydia prevalence compared to non-tailored interventions. The most effective intervention was the tailored condom intervention, which was assumed to result in a relative reduction in chlamydia prevalence of 18% versus 12% in the non-tailored scenario. Thus, it is important to assess multiple psychological and behavioural characteristics of individuals. Tailored interventions may be more successful in achieving risk-reducing behaviour, and consequently, reduce chlamydia prevalence more effectively.

Prevalence and determinants of genital Chlamydia trachomatis among school-going, sexually experienced adolescents in urban and rural Indigenous regions of Panama

Objectives

To determine the prevalence and risk factors of genital Chlamydia trachomatis (CT) among school-going sexually experienced male and female adolescents in Panama.

Methods

We conducted two multisite cross-sectional studies using two-stage cluster sampling to select adolescents aged 14–19 years attending urban public high schools (URB) in Panama City, San Miguelito, Colón and Panama Oeste from 2015 to 2018, and in the rural Indigenous Comarca Ngäbe-Buglé (CNB) from July–November 2018. CT testing was performed by real-time PCR on urine samples. Random-effects logistic regression accounting for sample clustering was used to identify risk factors.

Results

We enrolled 3166 participants (54.3% females), median age 17 years (IQR: 15.9–18.1), with no difference by sex. Sexual experience was reported by 1954 (61.7%) participants. Combined CT prevalence was 15.8% (95% CI: 14.2 to 17.4), with no significant differences by region (URB=16.5%, 95% CI: 14.7% to 18.6%; CNB=13.6%, 95% CI: 10.9% to 16.8%; p=0.12). In an age-and-region-adjusted analysis, CT prevalence was higher among female participants compared with males (21.6% vs 9.1%, adjusted OR (AOR)=2.87, 95% CI: 1.62 to 5.10). Among sexually experienced females, CT prevalence was higher among those who reported ≥3 lifetime sex partners compared with one partner (33.5% vs 15.3%, AOR=2.20, 95% CI: 1.09 to 4.07); and among those reporting at least one pregnancy compared with nulligravidae participants (30.9% vs 13.8%, AOR=1.89, 95% CI: 1.05 to 3.43). In unadjusted analyses among males, CT was associated with older age (11.5% among those aged 18–19 years vs 3.4% among those aged 14–15 years, OR=3.69, 95% CI: 1.10 to 12.33).

Conclusions

We report high CT prevalence among sexually experienced, school-going adolescents in Panama. Female adolescents, particularly those with multiple sex partners and a history of pregnancy, were at highest risk. Adolescent-targeted CT screening should be implemented in Panama. Additionally, evidence-based comprehensive sexuality education will be imperative.

Double trouble: modelling the impact of low risk perception and high-risk sexual behaviour on chlamydia transmission

Risk perception plays an important role in testing behaviour for sexually transmitted infections, but is rarely included in mathematical models exploring the impact of testing. We explored the impact of incorporating sexual behaviour (SB), risk perception (RP) and differential testing uptake in SB-RP groups on prevalence, using chlamydia as an example. We developed a pair model with a susceptible-infected-susceptible structure representing heterosexuals aged 16-26 years. The effect of testing on chlamydia prevalence was compared between a model with only SB (SB model) and a model with SB and RP (SB-RP model). In the SB-RP model, a scenario without differential testing uptake in SB-RP groups was compared to scenarios with differential testing uptake in SB-RP groups. Introducing testing into the SB-RP model resulted in a slightly smaller reduction in chlamydia prevalence (-38.0%) as compared to the SB model (-40.4%). In the SB-RP model, the scenario without differential testing uptake in SB-RP groups overestimated the reduction in chlamydia prevalence (with 4.8%), especially in the group with high SB and low RP (19.8%). We conclude that mathematical models incorporating RP and differential testing uptake in SB-RP groups improve the impact assessment of testing and treatment on chlamydia prevalence.

Discrepancies between observed data and predictions from mathematical modelling of the impact of screening interventions on Chlamydia trachomatis prevalence

Mathematical modelling studies of C. trachomatis transmission predict that interventions to screen and treat chlamydia infection will reduce prevalence to a greater degree than that observed in empirical population-based studies. We investigated two factors that might explain this discrepancy: partial immunity after natural infection clearance and differential screening coverage according to infection risk. We used four variants of a compartmental model for heterosexual C. trachomatis transmission, parameterized using data from England about sexual behaviour, C. trachomatis testing, diagnosis and prevalence, and Markov Chain Monte Carlo methods for statistical inference. In our baseline scenario, a model in which partial immunity follows natural infection clearance and the proportion of tests done in chlamydia-infected people decreases over time fitted the data best. The model predicts that partial immunity reduced susceptibility to reinfection by 68% (95% Bayesian credible interval 46-87%). The estimated screening rate was 4.3 (2.2-6.6) times higher for infected than for uninfected women in 2000, decreasing to 2.1 (1.4-2.9) in 2011. Despite incorporation of these factors, the model still predicted a marked decline in C. trachomatis prevalence. To reduce the gap between modelling and data, advances are needed in knowledge about factors influencing the coverage of chlamydia screening, the immunology of C. trachomatis and changes in C. trachomatis prevalence at the population level.

The Impact of Screening and Partner Notification on Chlamydia Prevalence and Numbers of Infections Averted in the United States, 2000-2015: Evaluation of Epidemiologic Trends Using a Pair-Formation Transmission Model

Population-level effects of control strategies on the dynamics of Chlamydia trachomatis transmission are difficult to quantify. In this study, we calibrated a novel sex- and age-stratified pair-formation transmission model of chlamydial infection to epidemiologic data in the United States for 2000–2015. We used sex- and age-specific prevalence estimates from the National Health and Nutrition Examination Surveys, case report data from national chlamydia surveillance, and survey data from the Youth Risk Behavior Survey on the proportion of the sexually active population aged 15–18 years. We were able to reconcile national prevalence estimates and case report data by allowing for changes over time in screening coverage and reporting completeness. In retrospective analysis, chlamydia prevalence was estimated to be almost twice the current levels in the absence of screening and partner notification. Although chlamydia screening and partner notification were both found to reduce chlamydia burden, the relative magnitude of their estimated impacts varied in our sensitivity analyses. The variation in the model predictions highlights the need for further data collection and research to improve our understanding of the natural history of chlamydia and the pathways through which prevention strategies affect transmission dynamics.

Changes in chlamydia prevalence and duration of infection estimated from testing and diagnosis rates in England: a model-based analysis using surveillance data, 2000-15

BACKGROUND

Chlamydia screening programmes have been implemented in several countries, but the effects of screening on incidence, prevalence, and reproductive sequelae remain unclear. In England, despite increases in testing with the rollout of the National Chlamydia Screening Programme (NCSP; 2003-08), prevalence estimated in 10-yearly population-based surveys was similar before (1999-2001) and after (2010-12) the programme. However, the precision of these previous estimates was limited by the low numbers of infections. We aimed to establish annual, rather than 10-yearly, estimates of chlamydia prevalence and infection duration.

METHODS

In this model-based analysis, we used previously published minimum and maximum estimates and Public Health England data for chlamydia test coverage and diagnoses in men and women aged 15-24 years in England, before, during, and after the scale-up of national chlamydia screening. We used a mechanistic model, which accounted for symptomatic chlamydia testing and asymptomatic screening, to estimate changes in prevalence and average duration of infections for each year. We describe estimates derived from the maximum and minimum numbers of tests and diagnoses as maximum and minimum estimates, regardless of their relative magnitude.

FINDINGS

The data included numbers of tests and diagnoses in men and women aged 15-19 years and 20-24 years in England each year from 2000 to 2015. We estimated reductions in prevalence and average infection duration in both sexes once screening was fully implemented. From 2008 to 2010, estimated posterior median prevalence reductions in people aged 15-24 years were 0·68 percentage points (95% credible interval 0·26-1·40; minimum) and 0·66 percentage points (0·25-1·37; maximum) for men and 0·77 percentage points (0·45-1·27) for women (minimum and maximum estimates were the same for women). Over the same time period, mean duration of infection reduced by 75 days (95% credible interval 17-255; minimum) and 74 days (95% credible interval 17-247; maximum) in men and 30 days (22-40) in women. Since 2010, some of the progress made by the NCSP has been reversed, alongside a reduction in testing.

INTERPRETATION

Our analysis provides the first evidence for a reduction in chlamydia prevalence in England concurrent with large-scale population testing. It also shows a consistent decline in the average duration of infections, which is a measure of screening effectiveness that is unaffected by behavioural changes.

FUNDING

National Institute for Health Research, Medical Research Council.

Prevalence of Mycoplasma genitalium in different population groups: systematic review andmeta-analysis

BACKGROUND

Mycoplasma genitalium is a common cause of non-gonococcal non-chlamydial urethritis and cervicitis. Testing of asymptomatic populations has been proposed, but prevalence in asymptomatic populations is not well established. We aimed to estimate the prevalence of M. genitalium in the general population, pregnant women, men who have sex with men (MSM), commercial sex workers (CSWs) and clinic-based samples, METHODS: We searched Embase, Medline, IndMED, African Index Medicus and LILACS from 1 January 1991 to 12 July 2016 without language restrictions. We included studies with 500 participants or more. Two reviewers independently screened and selected studies and extracted data. We examined forest plots and conducted random-effects meta-analysis to estimate prevalence, if appropriate. Between-study heterogeneity was examined using the I2 statistic and meta-regression.

RESULTS

Of 3316 screened records, 63 were included. In randomly selected samples from the general population, the summary prevalence was 1.3% (95% CI 1.0% to 1.8%, I2 41.5%, three studies, 9091 people) in countries with higher levels of development and 3.9% (95% CI 2.2 to 6.7, I2 89.2%, three studies, 3809 people) in countries with lower levels. Prevalence was similar in women and men (P=0.47). In clinic based samples, prevalence estimates were higher, except in asymptomatic patients (0.8%, 95% CI 0.4 to 1.4, I2 0.0%, three studies, 2889 people). Summary prevalence estimates were, in the following groups: pregnant women 0.9% (95% CI 0.6% to 1.4%, I2 0%, four studies, 3472 people), MSM in the community 3.2% (95% CI 2.1 to 5.1, I2 78.3%, five studies, 3012 people) and female CSWs in the community 15.9% (95% CI 13.5 to 18.9, I2 79.9%, four studies, 4006 people).

DISCUSSION

This systematic review can inform testing guidelines for M. genitalium. The low estimated prevalence of M. genitalium in the general population, pregnant women and asymptomatic attenders at clinics does not support expansion of testing to these groups.

REGISTRATION NUMBERS

PROSPERO: CRD42015020420.

From Experiment to Theory: What Can We Learn from Growth Curves?

Finding an appropriate functional form to describe population growth based on key properties of a described system allows making justified predictions about future population development. This information can be of vital importance in all areas of research, ranging from cell growth to global demography. Here, we use this connection between theory and observation to pose the following question: what can we infer about intrinsic properties of a population (i.e., degree of heterogeneity, or dependence on external resources) based on which growth function best fits its growth dynamics? We investigate several nonstandard classes of multi-phase growth curves that capture different stages of population growth; these models include hyperbolic-exponential, exponential-linear, exponential-linear-saturation growth patterns. The constructed models account explicitly for the process of natural selection within inhomogeneous populations. Based on the underlying hypotheses for each of the models, we identify whether the population that it best fits by a particular curve is more likely to be homogeneous or heterogeneous, grow in a density-dependent or frequency-dependent manner, and whether it depends on external resources during any or all stages of its development. We apply these predictions to cancer cell growth and demographic data obtained from the literature. Our theory, if confirmed, can provide an additional biomarker and a predictive tool to complement experimental research.