Comparison of Method Group Precision in Proficiency Testing of Clinical Chemistry Tests Before and After Two Major Changes: Reorganization of Proficiency Testing and Implementation of the Differential Benefit for the Quality of Laboratory Tests.

Dear Editor,

We had an impression from our practice that recent proficiency testing (PT) results of our laboratory tend to have higher standard deviation indexes (SDIs) than those previously reported, even for similar differences from group means, i.e., error. The performance of analytical systems, for example, with respect to precision, has improved consistently over time owing to improving technology. However, two major changes in relation to PT have recently been implemented in Korea [12]. First, in early 2016, the Korean Association of External Quality Assessment Service (KEQAS, previously KAQACL) reorganized the PT system. The input of measurement results and the release of PT reports are processed through only the KEQAS PT website, http://eqas.keqas.org/ [3]. Moreover, in the revised PT report format, the results of an individual laboratory are overlaid with histograms based on data from all PT participant laboratories, enabling comparison of the distribution of results [1]. These changes have resulted in more informative and efficient feedback to participant laboratories. Second, in the second half of 2017, the Differential Benefit for the Quality of Laboratory Tests (DBQLT) program was implemented by the Ministry of Health and Welfare and the Health Insurance Review and Assessment Service; it is applicable to all hospitals with a clinical laboratory in Korea [2]. DBQLT scores in terms of PT range from 0 to 25 points based on the return rate and the percentage of correct results. Although these two changes were expected to affect the analytical performance of laboratory tests as well as PT achievement in clinical laboratories, these effects have not been studied. Therefore, we examined whether analytical performance, as determined by the precision of method groups in clinical chemistry tests, differed significantly before and after PT reorganization and DBQLT implementation.

The KEQAS PT program for general clinical chemistry shipped PT materials in four episodes in a year. In an episode, there were three challenges with the corresponding number of PT materials. Total participant summaries for 30 episodes (90 challenges) during the period between the first episode (CC-11-01 to CC-11-03) in May 2011 and the second episode (CC-18-04 to CC-18-06) in April 2018 were downloaded from the KEQAS PT website [3]. The CV (%) for each of the 28 analyte group combinations were analyzed; one or two dominant method groups were selected for each of the following 19 analytes: ALT, albumin, alkaline phosphatase, AST, total bilirubin, calcium, chloride, total cholesterol, creatinine, gamma-glutamyl transferase, glucose, lactate dehydrogenase, phosphorus, potassium, total protein, sodium, triglyceride, urea nitrogen, and uric acid.

The data covered three periods: the first included years from 2011 to 2015, before the reorganization of PT (denoted as BR). The years in this period were termed BR-1 to BR-5. After reorganization of PT (AR) in early 2016, six episodes before implementation of DBQLT (AR-BD) comprised the second period, and the next four episodes after implementation of DBQLT (AR-AD) comprised the third period. CVgroup, representing the percentage error equivalent to an SDI of 1.0 for an analyte, was compared between BR-1 and BR-5, and then between BR, AR-BD, and AR-AD. Data were summarized as mean and SD. The Kruskal–Wallis rank sum test was used for comparisons among periods, and post-hoc Dunn's test with Bonferroni adjustment was used for multiple comparisons. Analyses were performed with R 3.3.2 [4]. This study was exempted from approval by the Institutional Review Board of the Catholic Medical Center, The Catholic University of Korea (HC18ZESE0071).

Compared with that of BR-1, CVgroup of BR-5 decreased in five analyte groups (5/28, 17.9%) and increased in two (2/28, 7.1%) (Table 1). In the transition from BR to AR-BD, CVgroup decreased in eight analyte groups (8/28, 28.6%) and increased in five analyte groups (5/28, 17.9%). In the transition from AR-BD to AR-AD, CVgroup decreased in five analyte groups (5/28, 17.9%) and increased in one (1/28, 3.6%). Between BR and AR-AD, CVgroup increased and decreased in another two analyte groups, respectively. Overall, throughout the PT reorganization and DBQLT implementation, the precision of 14 analyte groups ([8+5+1]/28, 50.0%) improved, and CVgroup of seven analyte groups ([5+1+1]/28, 25.0%) increased (Table 2).

Table 1

CVgroup (mean±SD, %) for 28 analyte groups in 2011–2015, before the reorganization of proficiency testing

Period	BR-1		BR-2		BR-3		BR-4		BR-5		P among BR-1 to BR-5*	Change in CV between two consecutive periods†	Pairwise P‡
Analyte: Group AR (Group BR)	N (fraction, %)	CVgroup (%)	N (fraction, %)	CVgroup (%)	N (fraction, %)	CVgroup (%)	N (fraction, %)	CVgroup (%)	N (fraction, %)	CVgroup (%)			BR-1 vs BR-2	BR-2 vs BR-3	BR-3 vs BR-4	BR-4 vs BR-5	BR-1 vs BR-5
ALT: LDH without P5P (UV without P5P)	1,154.0±0.0 (93.7±0.0)	5.4±0.5	1,075.0±24.9 (94.0±0.3)	6.2±0.9	1,115.0±21.9 (93.5±0.4)	6.8±1.0	1,173.0±47.2 (93.4±0.5)	6.3±1.0	1,198.0±43.7 (93.6±0.6)	6.2±1.1	0.007	++−−	0.121	0.716	0.834	1.000	0.172
Albumin: Dye Binding-BCG (Dye Binding-BCG)	1,115.0±0.0 (96.9±0.0)	4.0±0.3	1,050.0±25.3 (97.6±0.1)	4.0±0.4	1,087.0±21.6 (96.7±0.1)	4.1±0.4	1,136.0±46.8 (97.0±0.1)	4.4±0.9	1,158.0±41.0 (96.9±0.3)	4.2±0.4	0.628	+++−	1.000	1.000	1.000	1.000	1.000
Alkaline phosphatase: PNPP, AMP buffer (PNPP, AMP buffer)	554.0±0.5 (50.2±0.0)	14.4±1.2	466.0±11.2 (44.5±0.3)	15.0±1.4	504.0±7.6 (46.8±0.3)	15.6±2.3	555.0±14.2 (49.3±0.9)	16.1±2.7	597.0±18.3 (52.1±0.6)	16.4±3.7	0.615	++++	1.000	1.000	1.000	1.000	1.000
Alkaline phosphatase: PNPP, EAE buffer (PNPP, EAE buffer)	330.0±0.0 (29.9±0.0)	6.1±0.1	331.0±8.0 (31.6±0.7)	6.4±1.0	336.0±8.9 (31.2±0.2)	6.8±0.5	335.0±15.4 (29.7±0.2)	6.9±0.7	338.0±11.1 (29.5±0.2)	9.3±2.0	<0.001	++++	0.910	0.862	1.000	0.085	<0.001
AST: MDH without P5P (UV without P5P)	1,153.0±0.0 (93.7±0.0)	5.1±0.5	1,074.0±25.4 (94.0±0.2)	5.8±1.1	1,116.0±21.8 (93.7±0.4)	5.7±1.0	1,172.0±46.8 (93.5±0.5)	5.4±0.7	1,198.0±43.2 (93.7±0.6)	5.9±1.2	0.262	+−−+	0.361	1.000	1.000	1.000	0.415
Bilirubin, total: Diazonium salt-Surfactant with blank (Diazonium salt/Diazonium ion with blank; Diazo: stable diazonium salts)	267.0±0.0 (23.3±0.0)	8.3±1.5	213.0±5.8 (19.8±0.5)	9.2±2.9	244.0±2.1 (21.8±0.5)	9.2±3.0	260.0±5.4 (22.3±0.9)	8.7±2.1	290.0±12.5 (24.3±0.2)	7.7±2.3	0.657	+−−−	1.000	1.000	1.000	1.000	1.000
Bilirubin, total: Vanadate oxidation (Vanadate oxidation)	391.0±0.0 (34.1±0.0)	5.7±0.6	316.0±8.6 (29.4±0.3)	5.7±1.3	339.0±6.2 (30.2±0.2)	7.0±3.9	380.0±11.3 (32.5±1.1)	6.0±0.7	404.0±13.0 (33.8±0.3)	6.3±0.9	0.223	++−+	1.000	1.000	1.000	1.000	0.438
Calcium: Colorimetry Arsenazo III (Arsenazo III Dye)	101.0±0.0 (16.3±0.0)	2.8±0.1	89.5±4.5 (15.1±0.7)	3.7±0.7	92.8±2.9 (15.3±0.4)	3.7±1.3	99.8±3.4 (15.9±0.3)	3.0±0.5	111.0±2.5 (17.1±0.2)	3.1±0.4	0.001	+−−+	0.001	1.000	0.226	1.000	0.766
Calcium: Colorimetry_OCPC (Cresolphthalein complexone)	449.0±0.0 (72.4±0.0)	3.1±0.3	479.0±8.8 (80.8±0.6)	3.3±0.4	476.0±13.2 (78.3±0.8)	3.2±0.4	461.0±13.9 (73.5±0.7)	3.0±0.4	454.0±7.7 (70.2±0.3)	2.7±0.3	0.001	+−−−	0.618	1.000	0.487	0.597	0.119
Chloride: ISE, diluted (ISE, diluted [indirect])	457.0±0.0 (58.1±0.0)	2.7±0.7	402.0±7.3 (54.9±1.3)	2.3±0.7	426.0±6.2 (55.3±0.7)	2.3±0.8	455.0±6.0 (56.9±1.3)	2.5±0.8	481.0±9.9 (58.3±0.8)	2.5±0.7	0.309	−++−	0.386	1.000	1.000	1.000	1.000
Chloride: ISE, undiluted (ISE, undiluted [direct])	314.0±0.0 (39.9±0.0)	3.1±0.5	314.0±13.8 (42.8±1.3)	3.2±0.8	326.0±8.4 (42.4±0.6)	2.9±0.6	328.0±20.6 (40.9±1.2)	3.0±0.7	326.0±15.6 (39.4±0.8)	2.8±0.7	0.671	+−+−	1.000	1.000	1.000	1.000	1.000
Cholesterol, total: Cholesterol esterase (Enzymatic)	1,202.0±0.5 (98.8±0.0)	3.4±0.2	1,116.0±26.0 (98.9±0.1)	3.5±0.6	1,157.0±25.4 (98.3±0.2)	3.5±0.5	1,221.0±48.8 (98.5±0.4)	3.8±0.8	1,245.0±49.4 (98.7±0.3)	3.6±0.5	0.733	+++−	1.000	1.000	1.000	1.000	1.000
Creatinine: kinetic Jaffe without compensation (kinetic alkaline picrate Jaffe reaction)	851.0±0.0 (70.0±0.0)	7.1±1.8	841.0±22.8 (74.8±0.2)	7.6±2.2	843.0±25.6 (71.8±0.7)	7.6±2.5	869.0±41.6 (70.2±0.4)	7.8±3.3	871.0±36.6 (69.1±0.1)	7.8±2.0	0.692	++++	1.000	1.000	1.000	1.000	0.828
Gamma-glutamyl transferase: γ-glutamyl-carboxy-nitroanilide (γ-glutamyl-carboxy-nitroanilide [IFCC, 37℃])	995.0±0.0 (82.6±0.0)	5.8±0.6	896.0±18.0 (80.1±0.6)	5.7±0.7	940.0±19.5 (80.7±0.2)	6.2±1.7	1,008.0±34.3 (82.2±0.8)	5.9±0.6	1,050.0±37.3 (84.0±0.6)	7.3±5.2	0.921	−+−+	1.000	1.000	1.000	1.000	1.000
Glucose: Glucose oxidase (Glucose oxidase [Colorimetric])	276.0±0.0 (22.8±0.0)	4.1±0.0	246.0±10.3 (22.1±0.4)	4.4±0.6	255.0±8.9 (21.9±0.4)	4.4±0.7	286.0±15.2 (23.2±0.3)	4.4±0.4	289.0±14.1 (22.9±0.2)	4.4±0.5	0.692	++−−	1.000	1.000	1.000	1.000	0.875
Glucose: Hexokinase (Hexokinase, UV)	886.0±0.0 (73.1±0.0)	2.8±0.3	827.0±14.9 (74.4±0.6)	3.1±0.4	856.0±13.2 (73.5±0.4)	3.2±0.6	893.0±28.9 (72.5±0.7)	3.0±0.3	924.0±31.3 (73.2±0.7)	3.1±0.4	0.353	++−+	0.390	1.000	1.000	1.000	0.366
Lactate dehydrogenase: Lactate to pyruvate (Lactate to pyruvate)	357.0±0.0 (43.5±0.0)	6.4±0.1	320.0±8.7 (40.2±0.6)	6.1±0.7	342.0±3.7 (42.3±0.2)	5.7±0.4	359.0±10.7 (42.9±0.4)	6.0±0.4	370.0±7.5 (44.0±0.3)	5.9±0.5	0.005	−−+−	0.134	0.621	0.628	1.000	0.040
Lactate dehydrogenase: Pyruvate to lactate (Pyruvate to lactate)	439.0±0.0 (53.5±0.0)	5.7±0.1	454.0±7.8 (57.0±0.6)	6.0±0.6	441.0±8.6 (54.6±0.2)	7.9±2.3	450.0±15.4 (53.8±0.3)	7.0±2.0	445.0±13.4 (52.8±0.2)	6.5±1.0	< 0.001	++−−	0.906	0.018	0.721	1.000	0.077
Phosphorus: Phosphomolybdate (Phosphomolybdate, UV)	435.0±0.0 (74.7±0.0)	3.8±0.4	412.0±4.6 (73.6±0.3)	4.0±1.1	420.0±7.6 (74.2±0.4)	3.9±0.9	439.0±8.3 (74.8±0.4)	4.0±0.6	457.0±8.1 (75.2±0.2)	3.3±0.5	0.011	+−+−	1.000	1.000	1.000	0.009	0.029
Potassium: ISE, diluted (ISE, diluted [indirect])	461.0±0.0 (57.5±0.0)	1.7±0.2	410.0±6.9 (54.9±1.3)	1.8±0.4	432.0±5.8 (55.2±0.7)	1.8±0.3	460.0±6.2 (56.5±1.2)	2.1±1.0	486.0±9.9 (57.9±0.8)	1.8±0.2	0.631	+−+−	1.000	1.000	1.000	1.000	1.000
Potassium: ISE, undiluted (ISE, undiluted [direct])	325.0±0.0 (40.5±0.0)	3.5±0.1	318.0±14.2 (42.6±1.3)	3.0±0.3	330.0±8.4 (42.2±0.5)	2.9±0.3	337.0±19.6 (41.3±1.0)	3.0±0.4	333.0±16.9 (39.7±0.9)	3.0±0.3	0.001	−−+−	0.004	1.000	1.000	1.000	0.002
Protein, total: Biuret method (Biuret method)	1,131.0±0.0 (99.0±0.0)	2.9±0.1	1,062.0±24.6 (99.3±0.1)	3.3±0.4	1,106.0±23.9 (98.9±0.2)	3.6±0.4	1,152.0±46.6 (99.0±0.2)	3.7±0.6	1,173.0±41.8 (98.9±0.2)	3.3±0.4	0.001	+++−	0.067	1.000	1.000	1.000	0.049
Sodium: ISE, diluted (ISE, diluted [indirect])	461.0±0.0 (57.5±0.0)	1.4±0.1	409.0±6.4 (54.8±1.3)	1.4±0.2	431.0±6.0 (55.1±0.7)	1.3±0.3	460.0±6.0 (56.4±1.3)	1.3±0.2	486.0±9.5 (57.9±0.8)	1.3±0.2	0.099	−−+−	0.877	1.000	1.000	1.000	0.090
Sodium: ISE, undiluted (ISE, undiluted [direct])	325.0±0.0 (40.5±0.0)	2.3±0.1	317.0±13.7 (42.4±1.2)	2.1±0.4	329.0±8.6 (42.0±0.6)	1.8±0.3	337.0±19.3 (41.3±1.0)	1.9±0.4	334.0±16.9 (39.7±0.9)	1.8±0.4	0.004	−−+−	0.982	0.391	1.000	1.000	0.004
Triglyceride: Enzymatic with free glycerol elimination (Enzymatic [glycerol phosphate oxidase] with glycerol blank with sample blank)	342.0±0.0 (28.6±0.0)	13.2±3.8	336.0±6.2 (30.3±0.4)	7.3±4.1	333.0±13.2 (28.8±0.5)	8.0±4.0	342.0±7.9 (28.1±0.7)	8.2±4.7	355.0±12.0 (28.5±0.3)	7.5±4.2	0.016	−++−	0.016	1.000	1.000	1.000	0.020
Triglyceride: Enzymatic without free glycerol elimination (Enzymatic [glycerol phosphate oxidase] without glycerol blank without sample blank)	552.0±0.0 (46.2±0.0)	7.1±1.9	462.0±9.7 (41.6±0.8)	6.6±1.1	512.0±8.0 (44.2±0.8)	6.2±0.4	563.0±21.7 (46.2±0.3)	6.3±1.3	586.0±24.5 (47.1±0.5)	5.9±0.9	0.309	−−+−	1.000	1.000	1.000	1.000	0.555
Urea nitrogen: Urease with GLDH (Urease with GLDH [Coupled Enzymes])	1,132.0±0.0 (96.5±0.0)	5.3±0.2	1,061.0±23.9 (97.0±0.1)	4.7±0.7	1,102.0±24.5 (96.2±0.1)	5.0±0.8	1,154.0±46.5 (96.4±0.3)	5.0±0.4	1,180.0±45.2 (96.6±0.2)	4.9±0.6	0.124	−+−−	0.044	1.000	1.000	1.000	0.488
Uric acid: Uricase (Uricase)	989.0±0.0 (97.7±0.0)	3.7±0.4	953.0±21.1 (98.3±0.1)	4.2±0.6	973.0±20.7 (97.8±0.1)	4.0±0.6	1,007.0±38.6 (97.7±0.5)	3.9±0.3	1,032.0±33.2 (97.9±0.5)	3.9±0.4	0.276	+−−+	0.157	1.000	1.000	1.000	0.630

*P from Kruskal–Wallis rank sum test; †Direction of change (“+” indicates an increase and “−” indicates a decrease) between BR-1 and BR-2, between BR-2 and BR-3, between BR-3 and BR-4, and between BR-4 and BR-5; ‡P from Dunn's test.

Abbreviations: AR, after reorganization of proficiency testing; BR, before reorganization of proficiency testing; LDH, lactate dehydrogenase; P5P, pyridoxal-5′-phosphate; UV, ultraviolet; BCG, bromocresol green; PNPP, p-nitrophenyl phosphate; EAE, ethylaminoethanol; MDH, malate dehydrogenase; OCPC, o-cresolphthalein complexone; ISE, ion selective electrode; IFCC, International Federation of Clinical Chemistry and Laboratory Medicine; GLDH, glutamate dehydrogenase.

Table 2

CVgroup (mean±SD, %) for 28 analyte groups in 2011–2018, before and after the reorganization of proficiency testing and implementation of the differential benefit for the quality of laboratory tests

Period	BR		AR-BD		AR-AD		P among BR, AR-BD, and AR-AD*	Change in CV between two consecutive periods†	Pairwise P‡
Analyte: Group AR (Group BR)	N (fraction, %)	CVgroup (%)	N (fraction, %)	CVgroup (%)	N (fraction, %)	CVgroup (%)			BR vs AR-BD	AR-BD vs AR-AD	BR vs AR-AD
ALT: LDH without P5P (UV without P5P)	1,143.0±53.7 (93.6±0.4)	6.2±1.0	1,204.0±76.8 (87.5±4.5)	6.5±1.9	1,480.0±47.5 (93.2±1.2)	6.9±1.1	0.091	++	1.000	0.123	0.045
Albumin: Dye Binding-BCG (Dye Binding-BCG)	1,109.0±48.7 (97.0±0.3)	4.1±0.5	1,123.0±72.9 (88.6±4.8)	4.8±0.7	1,396.0±42.0 (95.3±1.0)	4.9±0.6	<0.001	++	<0.001	1.000	<0.001
Alkaline phosphatase: PNPP, AMP buffer (PNPP, AMP buffer)	535.0±47.4 (48.6±2.7)	15.5±2.5	613.0±36.6 (50.5±2.6)	17.5±2.7	779.0±32.7 (55.5±1.4)	15.2±1.5	0.004	+−	0.002	0.038	1.000
Alkaline phosphatase: PNPP, EAE buffer (PNPP, EAE buffer)	334.0±10.1 (30.4±0.9)	7.1±1.6	323.0±12.3 (26.5±0.7)	9.9±2.8	387.0±21.0 (27.5±1.0)	4.7±1.0	<0.001	+−	0.004	<0.001	<0.001
Aspartate Aminotransferase: MDH without P5P (UV without P5P)	1,142.0±53.8 (93.7±0.4)	5.6±0.9	1,205.0±78.1 (87.6±4.5)	5.4±1.1	1,484.0±47.6 (93.5±1.1)	5.8±1.4	0.697	−+	0.603	0.844	1.000
Bilirubin, total: Diazonium salt-Surfactant with blank (Diazonium salt/Diazonium ion with blank; Diazo: stable diazonium salts)	255.0±26.7 (22.3±1.6)	8.6±2.4	356.0±15.3 (28.0±0.9)	8.4±3.1	434.0±33.1 (29.4±1.8)	9.1±3.1	0.752	−+	0.752	0.765	1.000
Bilirubin, total: Vanadate oxidation (Vanadate oxidation)	366.0±34.4 (32.0±2.0)	6.1±1.9	436.0±21.2 (34.4±1.1)	6.2±0.7	563.0±29.2 (38.3±1.4)	4.6±0.9	<0.001	+−	0.174	<0.001	<0.001
Calcium: Colorimetry Arsenazo III (Arsenazo III Dye)	98.8±7.97 (15.9±0.8)	3.3±0.8	98.2±10.9 (14.9±1.7)	3.2±0.8	136.0±5.4 (19.2±0.4)	3.5±0.4	0.081	−+	1.000	0.118	0.039
Calcium: Colorimetry_OCPC (Cresolphthalein complexone)	464.0±15.5 (75.0±4.0)	3.1±0.4	387.0±24.8 (58.7±2.7)	2.6±0.4	414.0±5.9 (58.5±0.2)	2.7±0.6	<0.001	−+	<0.001	0.463	0.075
Chloride: ISE, diluted (ISE, diluted [indirect])	444.0±28.2 (56.7±1.7)	2.4±0.7	495.0±12.2 (57.9±0.7)	2.5±0.6	540.0±10.8 (54.3±0.3)	2.5±0.6	0.826	+−	0.876	1.000	1.000
Chloride: ISE, undiluted (ISE, undiluted [direct])	322.0±14.5 (41.1±1.6)	3.0±0.7	318.0±13.7 (37.2±0.6)	2.8±0.6	416.0±16.4 (41.8±0.6)	2.3±0.3	0.001	−−	0.546	0.013	<0.001
Cholesterol, total: Cholesterol esterase (Enzymatic)	1,188.0±57.7 (98.6±0.3)	3.6±0.6	1,206.0±81.0 (88.5±4.6)	3.2±0.7	1,494.0±49.4 (94.8±1.3)	3.5±0.9	0.016	−+	0.012	0.850	0.170
Creatinine: kinetic Jaffe without compensation (kinetic alkaline picrate [Jaffe reaction])	855.0±30.9 (71.2±2.1)	7.6±2.3	790.0±44.5 (61.0±1.8)	8.1±1.8	825.0±26.1 (53.7±0.4)	7.2±1.0	0.185	+−	0.100	0.530	0.964
Gamma-glutamyl transferase: Gamma-glutamyl-carboxy-nitroanilide (γ-Glutamyl-carboxy-nitroanilide [IFCC, 37℃])	978.0±59.9 (81.9±1.5)	6.2±2.5	915.0±38.9 (68.3±4.5)	6.9±1.2	1,212.0±266.0 (78.5±16.4)	6.8±1.0	<0.001	+−	0.001	1.000	0.005
Glucose: Glucose oxidase (Glucose oxidase [Colorimetric])	270.0±20.4 (22.6±0.6)	4.3±0.5	280.0±38.2 (20.5±2.6)	4.3±0.5	390.0±15.7 (25.0±0.5)	4.7±0.4	0.035	−+	1.000	0.059	0.016
Glucose: Hexokinase (Hexokinase, UV)	877.0±39.3 (73.4±0.8)	3.1±0.4	925.0±44.3 (67.9±2.1)	2.7±0.3	1,096.0±31.4 (70.1±0.6)	2.8±0.2	0.009	−+	0.005	0.651	0.180
Lactate dehydrogenase: lactate to pyruvate (Lactate to pyruvate)	350.0±18.7 (42.6±1.4)	6.0±0.5	380.0±17.1 (46.5±1.8)	5.2±0.6	445.0±10.8 (50.5±0.6)	5.0±0.6	<0.001	−−	<0.001	0.873	<0.001
Lactate dehydrogenase: pyruvate to lactate (Pyruvate to lactate)	446.0±11.7 (54.3±1.5)	6.6±1.6	349.0±23.2 (42.8±3.0)	6.6±1.2	388.0±10.9 (44.0±0.7)	7.6±1.0	0.002	++	0.680	0.024	0.001
Phosphorus: Phosphomolybdate (Phosphomolybdate, UV)	432.0±17.1 (74.5±0.6)	3.8±0.8	524.0±42.6 (85.8±7.5)	3.4±1.1	500.0±10.5 (75.8±0.5)	2.5±0.5	<0.001	−−	0.082	0.018	<0.001
Potassium: ISE, diluted (ISE, diluted [indirect])	450.0±27.1 (56.4±1.5)	1.8±0.5	498.0±12.1 (57.5±0.8)	1.9±0.5	542.0±10.4 (53.8±0.3)	1.6±0.6	0.357	+−	0.971	0.248	0.320
Potassium: ISE, undiluted (ISE, undiluted [direct])	329.0±14.9 (41.3±1.4)	3.1±0.4	328.0±14.1 (37.7±0.6)	2.6±0.9	427.0±17.2 (42.3±0.6)	2.6±0.3	<0.001	−−	<0.001	1.000	0.001
Protein, total: Biuret method (Biuret method)	1,125.0±49.6 (99.0±0.2)	3.4±0.5	1,080.0±98.6 (86.3±5.4)	3.1±0.4	1,384.0±41.4 (95.6±1.0)	3.3±0.6	0.553	−+	0.425	0.988	1.000
Sodium: ISE, diluted (ISE, diluted [indirect])	449.0±27.5 (56.3±1.6)	1.3±0.2	435.0±19.9 (50.2±2.1)	1.2±0.2	502.0±15.6 (49.8±0.5)	1.1±0.1	<0.001	−−	0.097	0.095	<0.001
Sodium: ISE, undiluted (ISE, undiluted [direct])	328.0±14.9 (41.2±1.3)	2.0±0.4	402.0±30.0 (46.3±3.0)	1.7±0.2	466.0±13.8 (46.2±0.3)	1.6±0.2	<0.001	−−	0.004	0.371	<0.001
Triglyceride: Enzymatic with free glycerol elimination (Enzymatic [glycerol phosphate oxidase] with glycerol blank with sample blank)	342.0±11.6 (28.9±0.9)	8.8±4.6	42.0±16.1 (33.0±0.6)	6.3±5.7	492.0±17.3 (32.0±0.5)	4.6±1.7	<0.001	−−	0.010	0.569	0.001
Triglyceride: Enzymatic without free glycerol elimination (Enzymatic [glycerol phosphate oxidase] without glycerol blank without sample blank)	535.0±46.7 (45.1±2.1)	6.4±1.3	747.0±63.6 (55.8±4.2)	5.6±2.5	965.0±32.9 (62.8±0.9)	4.9±1.1	<0.001	−−	<0.001	1.000	0.001
Urea nitrogen: Urease with GLDH (Urease with GLDH [Coupled Enzymes])	1,126.0±52.2 (96.5±0.3)	5.0±0.6	1,088.0±70.8 (83.4±4.1)	4.7±0.6	1,391.0±76.6 (91.6±3.5)	4.6±0.6	0.039	−−	0.112	0.906	0.050
Uric acid: Uricase (Uricase)	991.0±37.4 (97.9±0.4)	3.9±0.5	970.0±47.0 (89.4±4.1)	4.5±0.8	1,141.0±38.6 (94.8±1.5)	4.3±1.0	0.014	+−	0.005	0.231	0.636

*P from Kruskal–Wallis rank sum test; †Direction of change (“+” indicates an increase and “−” indicates a decrease) between BR and AR-BD and between AR-BD and AR-AD; ‡P from Dunn's test.

Abbreviations: AR, after reorganization of proficiency testing; BR, before reorganization of proficiency testing; BD, before implementation of the differential benefit for quality of laboratory tests; AD, after implementation of the differential benefit for quality of laboratory tests; LDH, lactate dehydrogenase; P5P, pyridoxal-5′-phosphate; UV, ultraviolet; BCG, bromocresol green; PNPP, p-nitrophenyl phosphate; EAE, ethylaminoethanol; MDH, malate dehydrogenase; OCPC, o-cresolphthalein complexone; ISE, ion selective electrode; IFCC, International Federation of Clinical Chemistry and Laboratory Medicine; GLDH, glutamate dehydrogenase.

A limitation of this study was that accuracy or trueness could not be considered. Despite a few reports on an association between analytical performance in PT and the accreditation of laboratories [56], this is the first comparison of the performance of method groups in clinical chemistry tests before and after recent changes in PT. Through PT reorganization and DBQLT implementation, the precision of analytical systems has improved. These results indicate that SDIs are likely to be more sensitive to measure precision in PT.