The Pennsylvania State University

Behavioral Endocrinology Lab Technical Notes
PSU Salivary Cortisol EIA Assay Performance Characteristics

Topical Outline

A. Recovery
B. Precision
    1. Inter-assay Coefficients of Variation
    2. Intra-assay Coefficients of Variation
C. Linearity of Dilution
D.Sensitivity
E. Correlation with Serum

Salivary Cortisol EIA Assay Performance Characteristics

A. Recovery

Two saliva samples containing different levels of endogenous cortisol were spiked with known quantities of cortisol and assayed.

Sample

Endogenous
(ug/dL)

Added
(ug/dL)

Expected
(ug/dL)

Observed
(ug/dL)

Recovery (%)

1

.41

.54

.95

.825

86.8%

 

  

 

.04

.450

.390

86.7%

2

.111

.54

.551

.614

111.4%

 

  

.04

.151

.136

90.1%

B. Precision

1. The intra-assay precision was determined from the mean of 10 replicates each.

Sample

N

Mean (ug/dL)

Standard
Deviation
(ug/dL)

Coefficient of Variation
(%)

H

10

.897

.01

3.88

M

10

.51

.03

6.22

L

10

.14

.01

7.12

2. The inter-assay precision was determined from the mean of average duplicates for ten separate runs.

Sample

N

Mean (ug/dL)

Standard
Deviation
(ug/dL)

Coefficient of Variation
(%)

H

10

.538

.04

6.69

L

10

.129

.01

6.88

C. Linearity of Dilution

Three saliva samples were diluted with PBS and assayed.

Sample

Dilution Factor

Expected (ug/dL)

Observed (ug/dL)

Recovery (%)

1

 

  

.513

 
 

1:2

.256

.271

105.8%

 

1:4

.128

.134

104.7%

 

1:8

.064

.057

89%
                1:16               .032               .036            112.5%  
                1:32               .016               .015              93.8%

2

 

 

.141

 
 

1:2

.071

.068

95.8%

 

1:4

.035

.035

100%

 

1:8

.018

.020

111.1%

3

 

  

.387

 
 

1:2

.193

.199

103.1%

 

1:4

.097

.100

103.1%

 

1:8

.048

.054

112.5%

 

1:16

.024

.023

95.8%
                1:32                .012               .011              91.7%

D. Sensitivity

The lower limit of sensitivity was determined by interpolating the mean minus 2 SD for 10 sets of duplicates at 0 ug/dL standard. The minimal concentration of cortisol that can be distinguished from 0 is < .007 ug/dL.

E. Correlation with Serum

The correlation between serum and saliva cortisol was determined by assaying 19 matched samples using the Diagnostic Products Corporation serum Coat-a-Count Cortisol RIA and the Salimetrics HS Salivary Cortisol EIA.

1. The correlation between saliva and serum was highly significant, r (17) = .960, p < .0001.

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