Adverse pregnancy outcomes attributable to syphilis infection continue to be a serious public health concern in many developing countries.1 The reported prevalence of syphilis among pregnant women in Sub-Saharan Africa has historically been relatively high, ranging from 3% to 17%.2–6 The disease may cause fetal or perinatal death in 40% of affected pregnancies7 and may also cause complications in surviving newborns, such as central nervous system abnormalities, deafness, multiple skin, bone, and joint deformities, and hematological disorders.8
Effective interventions to reduce or prevent adverse pregnancy outcomes should include antenatal serological screening and treatment using a single dose of intramuscular benzathine penicillin for those women whose sera tested positive.7 The recommended screening algorithm for syphilis involves 2 types of serologic tests (nontreponemal and treponemal). The use of a nontreponemal test alone, such as the rapid plasma reagin (RPR, Becton Dickinson, Sparks, MD) test, may result in some overtreatment (treatment of uninfected persons) owing to biologic false-positive reactions.7 On the other hand, the use of treponemal tests alone, such as the immunochromatographic strip assay (ICS, Abbott Laboratories, Chicago, IL), may also result in overtreatment because they do not differentiate between adequately treated and previous infections, but are relatively more sensitive.9 Additionally, treponemal tests may detect other nonsyphilitic treponemes, such as bejel and yaws, which are common in the tropics.10
Consequently, the 2-step algorithm was recommended.7,11,12 The 2-step algorithm involves screening with a nontreponemal test followed, if reactive by a confirmatory treponemal test such as the Treponema pallidum haemagglutination assay (TPHA, Fujirebio, Tokyo, Japan). This testing algorithm has been considered as the “gold standard” when assessing other tests and alternative screening algorithms.13–15 The reversed algorithm (i.e., treponemal followed by nontreponemal) has been shown to be less cost-effective in low and high prevalence settings.16
Although the RPR+TPHA algorithm is more specific than the use of the RPR test alone, it may result in significantly lower rates of treatment because these tests are usually conducted at laboratories remote from the clinical site and a substantial number of individuals fail to return for their test results and appropriate treatment.17 To overcome problems associated with failure to return for results, some researchers have advocated the use of on-site RPR testing performed by primary health care workers,18 whereas others have promoted the use of treponemal ICS.2
The New Dual Nontreponemal/Treponemal Point-of-Care Test
To combine the advantages offered by both serologic tests (nontreponemal and treponemal) and to reduce the time it takes for results to become available, an innovative dual treponemal/nontreponemal point-of-care immunochromatographic test (Dual-POC, Chembio Diagnostics Systems Inc, Medford, NY) for the simultaneous detection of reagin and treponemal antibodies has been developed and evaluated at the United States Centers for Disease Control and Prevention (CDC).19 The Dual-POC test is based on the principle of a dual path platform that has 2 antigens; 1 nontreponemal, 1 treponemal, and a control line striped onto the surface of a nitrocellulose membrane within the device. The Dual-POC test has the capacity to qualitatively detect nontreponemal antibodies (reagin) and confirm the specificity of reactive tests within 15 minutes. The test requires no expertise in reading test results.19
The main objective of this study was to estimate and compare the health and economic outcomes of 5 alternative prenatal serological screening protocol alternatives in Sub-Saharan Africa: onsite Dual-POC; onsite ICS; onsite RPR testing without confirmation; the conventional laboratory-based RPR+TPHA testing algorithm; and No-Program (which assumed no screening would take place). We estimated the adverse pregnancy outcomes prevented, the overtreatment rates, and the expected total costs from societal and health clinics' perspective. The results of this study may help provide important health and economic outcome information about the new testing technology when compared with existing strategies.
Performance of the Dual-POC
The results of a laboratory evaluation of the Dual-POC test conducted at the CDC indicated that the sensitivity and specificity of the nontreponemal line were 88.6% and 98.6%, respectively, when compared with the RPR test. The sensitivity and specificity of the treponemal test line of the Dual-POC test were found to be 96.5% and 95.5%, respectively, when compared with a T. pallidum passive particle agglutination assay (TPPA) which is equivalent to a TPHA test. The overall ability of the Dual-POC to detect both nontreponemal and treponemal antibodies in sera that were dually RPR and TPPA positive, (i.e., sensitivity) was 88.6%, while the ability of the test to exclude those sera that were not dually reactive (i.e., either RPR and/or TPPA negative), namely specificity, was 98.7%.19 Finally, the specificity of the Dual-POC test for previously treated infections was estimated to be 95.7%, following the CDC evaluation of the test.19
Study Model and Assumptions
Because the laboratory evaluation did not differentiate between the various stages of the disease, we assumed primary and secondary syphilis for those infected. We assumed that the screening program was undertaken at the first trimester of pregnancy for a cohort of 1000 pregnant women presenting in a Sub-Saharan African country with a historically high prevalence of syphilis (10% currently infected, and 15% previously infected and successfully treated). Thus, there were 1000 pregnancy outcomes for each alternative testing strategy. The analytic horizon assessed pregnancy outcomes attributable to untreated maternal syphilis infection (i.e., miscarriage in the second trimester, stillbirth, neonatal death, low birth weight, and congenital syphilis) and the natural history of undetected and therefore untreated primary and secondary syphilis infection for the mother. Cost included program costs and cost of pregnancy outcomes obtained from the literature. We assumed that the on-site tests (i.e., RPR, Dual POC, and ICS) were the basis for immediate subsequent treatment while the laboratory-based test (i.e., RPR+TPHA) assumed loss to follow-up before treatment. On the basis of published studies, we assumed that a single injection of 2.4 MU benzathine penicillin before the third trimester, which is used extensively in resource-poor settings, was 98% effective in treating early syphilis and therefore also effective in preventing vertical transmission of infection.11,20 Details of parameter values including sources are presented in Table 1. For the base-case analysis, we assumed that the cost of the Dual-POC test was the same as the ICS test ($3.73, 2008 US dollars).
We constructed a cohort decision analysis model to estimate and compare the total costs, number of women treated, and total number of adverse pregnancy outcomes prevented. Following previous studies,16,37 we computed overtreatment rates as the ratio of uninfected to infected women treated. Following recommended cost-effectiveness analyses guidelines,38 we computed cost as the net cost (total cost minus cost of No-Program). Negative net costs were reported as cost savings. We conducted several one-way sensitivity analyses as well as a multiway (Monte Carlo simulation) sensitivity analysis using the ranges provided in Table 1 to assess the relative health and economic outcomes. However, for the purpose of this study, we focused largely on the variables whose ranges potentially affected the relative costs and effectiveness of the Dual-POC strategy. In the Monte Carlo simulation, we focused on the Dual-POC and ICS results. We determined the disability-adjusted life years (DALYs) for both mother and child using Global Disease Burden measurement methods.39 To do this, we used separate Markov progression models based on Life Table values for South Africa and syphilis-related disability weights as provided in Table 1. An illustration of the Markov progression model showing the health states used to determine the DALYs for the mother is shown in Figure 1. We also repeated the analysis using global Life Table values.
We used the medical care component of the consumer price index for all urban consumers and adjusted all costs to 2008 United States dollars based on data from Sub-Saharan Africa.40 We used DATA Professional version 4.0 (TreeAge Software, Williamstown, MA) to construct the decision tree and conduct the comprehensive sensitivity analyses. Microsoft Excel, version 2007 (Microsoft Corporation, Redmond, WA), was used for summary analyses and presentation of results.
Health and Cost Outcomes
For a cohort of 1000 pregnancies, our model predicted a total of 39 adverse pregnancy outcomes for the No-Program strategy (total cost, $106,000); 13 for the lab-based RPR+TPHA (total cost, $86,000); 11 for the on-site RPR strategy (total cost, $84,000); 5 for the Dual-POC strategy (total cost, 79,000); and 2 for the ICS strategy (total cost, $76,000, see Table 2). Thus compared with the No-Program strategy, the ICS strategy prevented the most cases of adverse pregnancy outcomes, averted the most DALYs, and saved the highest costs, followed by the Dual-POC strategy (Tables 2, 3). The estimated total societal costs were slightly (<3%) higher than the total health clinic costs for all the screening strategies.
The results of our analyses are summarized in Table 2. Based on our assumptions and the base-case values used, no treatment was provided to uninfected women for the No-Program and laboratory-based RPR+TPHA strategies. Overtreatment rates were 0.32 (i.e., 23/71) for RPR, 0.18 (i.e., 16/89) for the Dual-POC, and 1.42 (i.e., 141/98) for ICS.
One-Way Sensitivity Analyses
In the one-way sensitivity analyses, we varied select variables and examined the total expected cost keeping all other variables constant. As expected, the relative test performance, test costs, and costs associated with adverse pregnancy outcomes were all influential. However, in almost all scenarios the ranges we used for the 1- and 2-way sensitivity analyses showed the screening programs to be cost-saving. When we increased the sensitivity of RPR from 60% to 98%, our results indicated that the RPR strategy was more cost-saving than the Dual-POC strategy when its sensitivity was at ≥85% and the ICS strategy when its sensitivity was at ≥93% (not shown). Figure 2 shows one-way sensitivity analyses results for syphilis prevalence, sensitivity of the Dual-POC test, cost of the Dual-POC test, and the sensitivity of the ICS test. The total cost for all the alternatives increased with syphilis prevalence (Fig. 2A). The Dual-POC strategy was less cost-saving than the onsite RPR strategy when the sensitivity of the Dual-POC test was less than 75%. However, the Dual-POC was the most cost-saving strategy when its sensitivity was ≥97% (Fig. 2B). Over the range of the cost of the Dual-POC test used (i.e., $0.5–$5), it remained more cost-saving than the onsite RPR and lab-based RPR+TPHA strategies (Fig. 2C). Finally, the sensitivity of the ICS test would have to be less than 90% for it to be less cost-saving than the Dual-POC strategy (Fig. 2D).
Multiway Sensitivity Analyses (Monte Carlo Simulation)
Using the ranges provided in Table 1 and assuming triangular distributions, we conducted a Monte Carlo simulation in which all variables (i.e., those with ranges) were varied at the same time for 1000 samples. The linear cost-saving trend line for ICS was higher than that for the Dual-POC ($13,000 vs. $9000).
We evaluated the health and economic outcomes of a new Dual-POC test compared with other diagnostic approaches, by examining expected pregnancy outcomes for 1000 women using conservative assumptions based on the performance characteristics of the test determined during an evaluation at the US CDC, serological profiles, and patient characteristics among antenatal clinic attendees previously reported in studies conducted in southern Africa.13,15,41 The laboratory based RPR+TPHA algorithm prevented 26 adverse pregnancy outcomes; the onsite RPR prevented 28; the Dual-POC prevented 34; and the ICS prevented 37 out of a total of 39 expected adverse pregnancy outcomes when there was no screening program. In terms of costs, the Dual-POC saved more cost than the onsite RPR and RPR+TPHA strategies, while the ICS strategy saved the most cost.
We checked the consistency of the results from our model by comparing our results with 2 recent studies that compared RPR+TPHA, RPR, and ICS for a similar setting.13,15 We found that the relative costs and effectiveness were consistent with their results based on the test performance values used. Additionally, the relative adverse pregnancy outcomes (miscarriage, congenital syphilis, low birth weight, stillbirth, and neonatal death) were consistent with the results reported in previous studies in similar settings.2,13,15
The strengths of this study are that we accounted for previously treated infections in the population that was studied.16,37 Second, we provided detail information on the expected overtreatment rate, which has been ignored (or has not been reported) in previously published studies. Finally, we improved the robustness of the results by conducting a comprehensive sensitivity analyses to account for inconsistencies in the data we used.
Our study has all the limitations associated with models in general: models are simplifications of reality and do not capture all the important characteristics of the events/phenomena being studied. In addition, the values used in our study were estimates obtained from other studies that were not consistent. There was no substantial difference between the total societal costs and total health clinic costs due to lack of reliable societal cost estimates. Another major limitation of our study is the inability to independently assess the Dual-POC. This was because the performance of the Dual-POC was assessed using RPR and TPPA (equivalent to TPHA) tests under ideal laboratory conditions. Also, we ignored ongoing transmission in the population. Nonetheless, to the extent that the screening strategy adopted does not affect the transmission dynamics, we believe that the relative costs and outcomes would not change for the onsite strategies (RPR, ICS, and Dual-POC). We did not account for costs associated with follow-ups, which may include assessment of response to therapy using serial quantitative titers.
The effect of transmission on the RPR+TPHA and No-Program strategies on our final results is difficult to assess because it would require more data and assumptions about mixing patterns and the prevalence among men. Additionally, pregnant women may not be an important source of ongoing transmission, which may limit the importance of this omission. Our assumption of untreated early syphilis (i.e., the exclusion of early and late latent forms of syphilis) in the hypothetical cohort of pregnant women implied that the estimated overall adverse health outcome was relatively higher for each strategy. However, it is difficult to determine how this omission affected the relative costs and effects of the testing strategies assessed in this study. Finally, due to lack of data, we did not include the cost of advanced stages of syphilis for the mothers.
With regard to comparing point-of-care tests to the 2-step algorithm, our results were consistent with previous studies in similar settings.2,13,15 Consistent with results reported by Rydzak and Goldie,15 our results indicated that the screening strategies examined (onsite RPR, ICS, lab-based RPR+TPHA, and onsite Dual-POC) were cost-saving when compared with the No-Program strategy. The cost-savings found in Rydzak and Goldie15 were substantially higher than we found because they accounted for lifetime pregnancies (6 per woman). Although we used a similar time frame and similar cost values as Blandford et al.,13 we included 2 other substantially more expensive adverse pregnancy outcomes (neonatal death and low birth weight) that may result from untreated maternal syphilis which resulted in higher savings for the screening strategies compared with the No-Program strategy.
Although the use of the RPR test may result in a small number of false positives,5 studies have demonstrated that nontreponemal test reactivity has a higher correlation with disease activity than detection of treponemal antibody.42 In addition, nontreponemal tests are more likely to be nonreactive following successful treatment.9 On the other hand, the use of a treponemal ICS test alone will inevitably result in high rates of overtreatment because all treponemal tests detect antibodies that may be present for a lifetime even following provision of adequate treatment.43 As demonstrated by our analyses, even when the specificity of the Dual-POC was as low as 85% (or 30% for previous infections), it resulted in a substantially lower overtreatment rate when compared with the ICS strategy.
Given these limitations of treponemal and nontreponemal tests (when used individually), the Dual-POC can help to detect syphilis cases in resource-poor settings while substantially reducing the rate of overtreatment. Additionally, the Dual-POC test should prove to be a useful screening/confirmatory test for the prevention of congenital disease because further laboratory evaluation indicated that its sensitivity was substantially higher (99.7%) for sera with RPR antibody titers ≥1:8 (during pregnancy, transplacental transmission of syphilis rarely occurs at confirmed maternal RPR titers <1:85,19). Further independent assessment of the Dual-POC should be undertaken to enable wider and more objective comparability, including guidelines on interpretation of results.
1.World Health Organization. The global elimination of congenital syphilis: Rationale and strategy for action [report]. Geneva, Switzerland: World Health Organization, 2007:38.
2.Bronzan RN, Mwesigwa-Kayongo DC, Narkunas D, et al. Onsite rapid antenatal syphilis screening with an immunochromatographic strip improves case detection and trip treatment in rural south African clinics. Sex Transm Dis 2007; 34:S55–S60.
3.Jenniskens F, Obwaka E, Kirisuah S, et al. Syphilis control in pregnancy-decentralization of screening facilities to primary-care level, a demonstration project in Nairobi, Kenya. Int J Gynecol Obstet 1995; 48:S121–S128.
4.Temmerman M, Gichangi P, Fonck K, et al. Effect of a syphilis control programme on pregnancy outcome in Nairobi, Kenya. Sex Transm Infect 2000; 76:117–121.
5.Watson-Jones D, Changalucha J, Gumodoka B, et al. Syphilis in pregnancy in Tanzania. I: Impact of maternal syphilis on outcome of pregnancy. J Infect Dis 2002; 186:940–947.
6.Watson-Jones D, Gumodoka B, Weiss H, et al. Syphilis in pregnancy in Tanzania. II: The effectiveness of antenatal syphilis screening and single-dose benzathine penicillin treatment for the prevention of adverse pregnancy outcomes. J Infect Dis 2002; 186:948–957.
7.Calonge N; United States Preventive Services Task Force. Screening for syphilis infection: Recommendation statement. Ann Fam Med 2004; 2:362–365.
8.Walker GJA, Walker DG. Congenital syphilis: A continuing but neglected problem. Semin Fetal Neonatal Med 2007; 12:198–206.
9.Centers for Disease Control and Prevention. Syphilis testing algorithms using treponemal tests for initial screening–four laboratories, New York City, 2005–2006. Morb Mortal Wkly Rep 2008; 57:872–875.
10.Larsen SA, Pope V, Johnson RE, et al, eds. A manual of tests for syphilis, 9th ed. Arlington, VA: American Public Health Association, 1998.
11.Centers for Disease Control and Prevention. Sexually transmitted disease treatment guidelines, 2006. MMWR Recomm Rep 2006; 55:1–94.
12.World Health Organization [WHO] & Scientific Group on Treponemal Infections. Treponemal Infections. Geneva, Switzerland: World Health Organization, 1982.
13.Blandford JM, Gift TL, Vasaikar S, et al. Cost-effectiveness of on-site antenatal screening to prevent congenital syphilis in rural Eastern Cape Province, Republic of South Africa. Sex Transm Dis 2007; 34:S61–S66.
14.Chuck A, Ohinmaa A, Tilley P, et al. Cost effectiveness of enzyme immunoassay and immunoblot testing for the diagnosis of syphilis. Int J STD AIDS 2008; 19:393–399.
15.Rydzak CE, Goldie SJ. Cost-effectiveness of rapid point-of-care prenatal syphilis screening in Sub-Saharan Africa. Sex Transm Dis 2008; 35:775–784.
16.Owusu-Edusei K Jr, Koski KA, Ballard RC. The tale of two serologic tests to screen for syphilis-treponemal and nontreponemal: Does the order matter? Sex Transm Dis 2011; 38:448–456.
17.Myer L, Wilkinson D, Lombard C, et al. Impact of on-site testing for maternal syphilis on treatment delays, treatment rates, and perinatal mortality in rural South Africa: A randomised controlled trial. Sex Transm Infect 2003; 79:208–213.
18.Delport SD, van den Berg JHY. On-site screening for syphilis at an antenatal clinic. S Afr Med J 1998; 88:43–44.
19.Castro AR, Esfandiari J, Kumar S, et al. Novel point-of-care test for simultaneous detection of nontreponemal and treponemal antibodies in patients with syphilis. J Clin Microbiol 2010; 48:4615–4619.
20.Watson-Jones D, Oliff M, Terris-Prestholt F, et al. Antenatal syphilis screening in Sub-Saharan Africa: Lessons learned from Tanzania. Trop Med Int Health 2005; 10:934–943.
21.Donders GGG, Desmyter J, Hooft P, et al. Apparent failure of one injection of benzathine penicillin G syphilis during pregnancy in human immunodeficiency virus-seronegative African women. Sex Transm Dis 1997; 24:94–101.
22.Garnett GP, Aral SO, Hoyle DV, et al. The natural history of syphilis. Implications for the transmission dynamics and control of infection. Sex Transm Dis 1997; 24:185–200.
23.Hook EW, Marra CM. Acquired syphilis in adults. N Engl J Med 1992; 326:1060–1069.
24.Sparling PF, Swartz MN, Musher DM, et al. Clinical manifestations of syphilis. In: Holmes KK, Sparling PF, Stamm WE, et al, eds. Sexually Transmitted Disease. New York, NY: McGraw Hill; 2008:661–684.
25.Clark EG, Danbolt N. The Oslo study of the natural history of untreated syphilis; an epidemiologic investigation based on a restudy of the Boeck-Bruusgaard material; a review and appraisal. Med Clin North Am 1964; 48:613.
26.Sheffield JS, Sanchez PJ, Morris G, et al. Congenital syphilis after maternal treatment for syphilis during pregnancy. Am J Obstet Gynecol 2002; 186:569–573.
27.Wendel GD, Sheffield JS, Hollier LM, et al. Treatment of syphilis in pregnancy and prevention of congenital syphilis. Clin Infect Dis 2002; 35:S200–S209.
28.Wicher V, Wicher K. Pathogenesis of maternal-fetal syphilis revisited. Clin Infect Dis 2001; 33:354–363.
29.Goldie SJ, Kuhn L, Denny L, et al. Policy analysis of cervical cancer screening strategies in low-resource settings: Clinical benefits and cost-effectiveness. JAMA 2001; 285:3107–3115.
30.Montoya PJ, Lukehart SA, Brentlinger PE, et al. Comparison of the diagnostic accuracy of a rapid immunochromatographic test and the rapid plasma reagin test for antenatal syphilis screening in Mozambique. Bull World Health Organ 2006; 84:97–104.
31.Malan AF, Ryan E, Vanderelst CW, et al. The cost of neonatal care. S Afr Med J 1992; 82:417–419.
32.Bateman DA, Phibbs CS, Joyce T, et al. The hospital cost of congenital syphilis. J Pediatr 1997; 130:752–758.
33.Koontz SL, de Perez OM, Leon K, et al. Treating incomplete abortion in El Salvador: Cost savings with manual vacuum aspiration. Contraception 2003; 68:345–351.
34.Murray CJL, Lopez AD, eds. The global burden of disease: A comprehensive assessment of mortality and disability from diseases, injuries and risk factors in 1990 and projected to 2020. Cambridge, MA: Harvard University Press, 1996.
35.Life Tables for WHO Member States - South Africa. Geneva, Switzerland: World Health Organization. Available at: http://apps.who.int/whosis/database/life_tables/life_tables_process.cfm?path=whosis,life_tables&language=english
. Accessed May, 2010.
36.World Population Prospects, the 2008 Revision: United Nations Department of Economic and Social Affairs. Available at: http://esa.un.org/unpd/wpp2008/peps_mortality-indicators-by-age.htm
. Accessed May, 2010.
37.Owusu-Edusei K Jr, Peterman TA, Ballard RC. Serologic testing for syphilis in the United States: A cost-effectiveness analysis of two screening algorithms. Sex Transm Dis 2011; 38:1–7.
38.Siegel JE, Weinstein MC, Russell LB, et al. Consensus statement: Recommendations for reporting cost-effectiveness analyses. Pediatr AIDS HIV Infect Fetus Adolesc 1997; 8:130–134.
39.Mathers CD, Vos T, Lopez AD, et al eds. National Burden of Disease Studies: A Practical Guide. Edition 2.0. Global Program on Evidence for Health Policy. Geneva, Switzerland: World Health Organization, 2001.
40.Consumer price indexes - all urban consumers. Washington, DC: United States Department of Labor. Available at: http://www.bls.gov/cpi/home.htm
. Accessed January 15, 2010.
41.Terris-Prestholt F, Watson-Jones D, Mugeye K, et al. Is antenatal syphilis screening still cost effective in Sub-Saharan Africa. Sex Transm Infect 2003; 79:375–381.
42.Aberle-Grasse J, Orton SL, Notari E, et al. Predictive value of past and current screening tests for syphilis in blood donors: Changing from a rapid plasma reagin test to an automated specific treponemal test for screening. Transfusion 1999; 39:206–211.
43.Lukehart SA. Biology of Treponemes. In: Holmes KK, Sparling PF, Stamm WE, et al eds. Sexually Transmitted Disease. New York, NY: McGraw Hill, 2008:647–659.