Table 4.
Comparison of Cortisol (nmol/L) Concentrations from QC Material processed in Glass, Vacuette™, PRT, RST™, and SST™ Tubes and Measured on the Immulite™ 1000 Analyzer
| Range of Assay Imprecision (CV%)a: 5.1–7.8% | ||||||||
|---|---|---|---|---|---|---|---|---|
| Tube Type | ||||||||
| Glass | Vacuette™ | PRT | RST™ | SST™ | USD | SCLd | Desirable Bias (%)e | |
| QC Level 1 (N=18) | ||||||||
| Mean (SD) | 131 (8.3) | 143 (8.3) | 147 (30.4) | 158 (16.3) | 154(11.6) | 15.2 | 111–168 | |
| Absolute difference (%)b | 12.0 (9.2) | 16.0 (12.2) | 27.0 (20.6)# | 23.0 (17.6)# | 12.5 | |||
| Pc (vs. Glass tubes) | 0.0002 | <0.0001 | <0.0001 | 0.0001 | ||||
| QC Level 2 (N=18) | ||||||||
| Mean (SD) | 635 (44.1) | 651 (39.7) | 650 (30.4) | 707 (42.5) | 706 (53.0) | 29.5 | 548–713 | |
| Absolute difference, (%) | 16.0 (2.5) | 15.0 (2.4) | 72.0 (11.3) | 71.0 (11.2) | 12.5 | |||
| Pc (vs. Glass tubes) | 0.260 | 0.234 | <0.0001 | <0.0001 | ||||
| QC Level 3 (N=18) | ||||||||
| Mean (SD) | 968 (47.2) | 995 (56.6) | 988 (66.2) | 1056 (79.5) | 1045 (93.8) | 52.7 | 789–1084 | |
| Absolute difference, (%) | 27.0 (2.8) | 20.0 (2.1) | 88.0 (9.0) | 77.0 (8.0)) | 12.5 | |||
| Pc (vs. Glass tubes) | 0.130 | 0.310 | <0.0003 | <0.0036 | ||||
a
Inter-assay imprecision across three levels of control materials.
b
Absolute difference and % change from glass tubes.
c
Probability Student paired t-test (two-sided) for mean difference between Vacuette™, PRT, RST™, and SST™ compared to glass tubes. P<0.01 is considered statistically significant and indicated in bold.
d
mean of quality material from plastic sample cup ±2.8 USD.
e
Maximum desirable bias (%) based on biological variation [6]. USD, usual standard deviation from QC data over a six-month period; SCL, significant change limit;
*
Exceeded SCL.
#
Exceeded maximum desirable bias.