Evaluation of Performance and False Positivity of Mediace RPR Test that Uses a Chemistry Autoanalyzer

Jaekwang Noh; Hak Hyun Ko; Yeomin Yun; Young Sook Choi; Sang Gon Lee; Sue Shin; Kyou Sup Han; Eun Young Song

doi:10.3343/kjlm.2008.28.4.312

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Noh, Ko, Yun, Choi, Lee, Shin, Han, and Song: Evaluation of Performance and False Positivity of Mediace RPR Test that Uses a Chemistry Autoanalyzer

Original Article

Korean J Leg Med 2008;28(4):312-318.

Published online: 14 February 2008

DOI: https://doi.org/10.3343/kjlm.2008.28.4.312

Evaluation of Performance and False Positivity of Mediace RPR Test that Uses a Chemistry Autoanalyzer

Jaekwang Noh, M.D.¹, Hak Hyun Ko, M.T.², Yeomin Yun, M.D.¹, Young Sook Choi, M.D.¹, Sang Gon Lee, M.D.³, Sue Shin, M.D.⁴, Kyou Sup Han, M.D.⁴, Eun Young Song, M.D.¹

¹Department of Laboratory Medicine, Konkuk University College of Medicine, Seoul, Korea

²Department of Laboratory Medicine, Konkuk University Hospital, Seoul, Korea

³Greencross Reference Laboratory, Yongin, Korea

⁴Department of Laboratory Medicine, Seoul National University College of Medicine, Seoul, Korea

Received 28 November 200722 February 2008 Accepted 22 February 2008

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

Background

We evaluated the performance and false positive rate of Mediace RPR test (Sekisui, Japan), a newly introduced nontreponemal test using a chemistry autoanalyzer.

Methods

The sensitivity of Mediace RPR test was analyzed using sera from 50 patients with syphilis in different stages (8 primary, 7 secondary, and 35 latent), 14 sera positive with fluorescent treponemal antibody absorption (FTA-ABS) IgM, and 74 sera positive with conventional rapid plasma regain (RPR) card test (Asan, Korea) and also positive with Treponema pallidum hemagglutination (TPHA) test or FTA-ABS IgG test. The specificity was analyzed on 108 healthy blood donors. We also performed RPR card test on 302 sera that had been tested positive with Mediace RPR test and also performed TPHA or FTA-ABS IgG test to analyze the false positive rate of Mediace RPR test. A cutoff value of 0.5 R.U. (RPR unit) was used for Mediace RPR test.

Results

Mediace RPR test on syphilitic sera of different stages (primary, secondary, and latent stages) and FTA-ABS IgM positive sera showed a sensitivity of 100%, 100%, 82.9% and 100%, respectively. Among the 74 sera positive with conventional RPR card test and TPHA or FTA-ABS IgG test, 55 were positive with Mediace test. The specificity of Mediace RPR test on blood donors was 97.2%. Among the 302 sera positive with Mediace RPR test, 137 sera (45.4%) were negative by RPR card and TPHA/FTA-ABS IgG tests.

Conclusions

Although the sensitivities of Mediace RPR were good for primary and secondary syphilis, due to its high negative rate of Mediace RPR over the conventional RPR positive samples, further studies are necessary whether it can replace conventional nontreponemal test for screening purpose. Moreover, in view of the high false positive rate, positive results by Mediace RPR test should be confirmed with treponemal tests.

Keywords: Mediace RPR, Sensitivity, Specificity, Cutoff, False positive, Syphilis, Non-treponemal test

REFERENCES

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Fig. 1.

ROC curve analysis of Mediace RPR test.

Fig. 2.

Linearity of Mediace RPR test

Table 1.

Sensitivity and specificity of Mediace RPR test with different cutoff values (1.0 R.U. vs 0.5 R.U.)

Specimen category (N of samples)	% Sensitivity cutoff 1.0/0.5	% Specificity cutoff 1.0/0.5
FTA-ABS IgM (+) sera (14)	100.0/100.0
Primary syphilis (8)	100.0/100.0
Secondary syphilis (7)	100.0/100.0
Latent syphilis
with history of tx. (15)	80.0/93.3
without history of tx. (20)	65.0/75.0
Blood donors (108)		100.0/97.2

Abbreviations: FTA-ABS, fluorescence treponema antibody-absorption; tx., treatment.

Table 2.

Analysis of false positive results of Mediace RPR test

Mediace RPR	RPR card	TPHA/FTA-ABS IgG	N of samples (%)	Possible interpretation
R	NR	R	5 (1.7)	Mediace FP or RPR card FN
R	NR	NR	137 (45.4)	Mediace FP/RPR card TN
R	R	R	94 (31.1)	Mediace TP/RPR card TP
R	R	NR	7 (2.3)	Mediace FP/RPR card FP
R	R	ND	59 (19.5)	ND

Abbreviations: R, reactive; NR, non-reactive; ND, not determined; Mediace, Mediace RPR; FP, false positive; TP, true positive; FN, false negative; TN, true negative.

Table 3.

Distribution of R.U. values of Mediace RPR-positive samples with possible interpretation (N=243)

Possible interpretation (N of samples)	0.5≤R.U.<1.0 N (%)	1.0≤R.U.<2.0 N (%)	R.U.≥2.0 N (%)
Mediace FP or RPR card FN (5)	2 (40.0)	1 (20.0)	2 (40.0)
Mediace FP/RPR card TN (137)	68 (49.6)	42 (30.7)	27 (19.7)
Mediace TP/RPR card TP (94)	16 (17.0)	14 (14.9)	64 (68.1)
Mediace FP/RPR card FP (7)	1 (14.3)	4 (57.1)	2 (28.6)

Abbreviations: R.U., RPR unit; Mediace, Mediace RPR; FP, false positive; TP, true positive; FN, false negative; TN, true negative.

Table 4.

Distribution of titers of RPR card test on Mediace RPR test negative and RPR card/TPHA or FTA-ABS IgG positive samples

Titer of RPR card	Cutoff 0.5 (N=19)	Cutoff 1.0 (N=32)
WR	6	8
1:1	4	6
1:2	6	13
1:4	3	5

Abbereviation: WR, weakly reactive at 1:1.

Table 5.

Precision of Mediace RPR test

	Low (1.0 R.U.)	Medium (4.5 R.U.)	High (8.0 R.U.)
Within-run CV (%)	8.3	3.3	1.6
Total CV (%)	12.8	3.5	1.7

Abbreviations: R.U., RPR unit; CV, coefficient of variant.

Table 6.

Changes of Mediace RPR test (R.U.) and the titer of RPR card after treatment

Stage of syphilis	No. case monitored	Pre-tx.		Post-tx. (3-6 months)		Post-tx. (9-12 months)
Stage of syphilis	No. case monitored	Mediace RPR^∗	RPR card	Mediace RPR^∗	RPR card	Mediace RPR^∗	RPR card
Primary	1	3.9	1:32	0.4	NT
	2	3.9	1:32	<0.1	NT
	3	2.4	NT	0.8	1:2
Secondary	1	2.1	>1:32	5.5	1:8
	2	2.6	>1:32	2.8	1:2
	3	2.8	1:32	0.9	1:1
	4	3.6	1:32	<0.1	NT
	5	4.6	1:8	NT	NT	0.2	NT
Latent
treated	1	3.8	1:32	4.2	1:8	2.6	1:8
untreated	1	4.0	1:32	1.8	1:1
	2	4.9	1:16	0.4	NT
	3	5.2	1:8	4.5	1:4
	4	4.4	NT	4.2	1:4	4.7	1:2
	5	2.3	1:32	7.0	1:16	4.0	1:2
	6	0.6	NT	0.7	WR

^∗ Samples with >3.0 R.U. were tested with serial dilutions and calculated from them.

Abbreviations: No., number; tx., treatment; RPR, rapid plasma reagin; NT, not tested; WR, weakly reactive at 1:1.

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