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Cortisol value calculations were conducted using two methods:
Results: As we see below, the formula used by Nist et al. results in negative values, which would mean that there is no cortisol in original samples.
| Summary | Nist et al. | My samples | Non-spiked only |
|---|---|---|---|
| Mean cort conc (pg/mg) | 23.74 | -0.18 | 7.9 |
| Range cort conc (pg/mg) | 2.1 to 124.9* | -29.3 to 11.76 | 2.71 to 11.76 |
| Weight range (mg) | 0.4 to 10.9 | 11 to 37.1 | 12 to 37 |
| Sample size | X | 30 | 18 |
This could be an artifact of an extremely high absorbance level. Non-spiked samples, however, result in values that are within the range found in similar studies of cortisol in human hair. After accounting for differences in dilution and weight, our results suggest some optimal parameters.
Conclusions:
Concerns
# DATA SET
current_test <- "Test3"
data_path <- file.path("./data", current_test)
# Define volume of methanol used for cortisol extraction
# vol added / vol recovered (mL)
extraction <- 1.3 / 1
# Reading of spike standard and conversion to ug/dL
std <- (3133 + 3146) / 2. # test 3 backfit
std.r <- std / 10000
Loading files and transforming units, including low quality data
df <- read.csv(file.path(data_path,"Data_QC_flagged.csv"))
kable(tail(df))
| Sample | Wells | Raw.OD | Binding.Perc | Conc_pg.ml | Ave_Conc_pg.ml | CV.Perc | SD | SEM | Category | Weight_mg | Buffer_nl | Spike | TotalVol_well_uL | SpikeVol_uL | Dilution | CV_categ | Binding.Perc_categ | Failed_samples | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 34 | 4 | F3 | 1.070 | 80.6 | 295.8 | 334.5 | 16.30 | 54.6 | 38.6 | NoSpike | 11.7 | 250 | 0 | 50 | 0 | 1 | HIGH CV | ABOVE 80% binding | HIGH CV;ABOVE 80% binding |
| 35 | 5 | G3 | 0.944 | 72.1 | 488.0 | 501.4 | 3.79 | 19.0 | 13.4 | NoSpike | 14.4 | 60 | 0 | 50 | 0 | 1 | NA | NA | NA |
| 36 | 6 | H3 | 0.490 | 34.7 | 2099.0 | 2204.0 | 6.75 | 149.0 | 105.0 | YesSpike | 13.7 | 60 | 1 | 50 | 25 | 1 | NA | NA | NA |
| 37 | 7 | A5 | 0.436 | 30.5 | 2551.0 | 2669.0 | 6.25 | 167.0 | 118.0 | YesSpike | 16.4 | 60 | 1 | 50 | 25 | 1 | NA | NA | NA |
| 38 | 8 | B5 | 1.030 | 77.8 | 354.9 | 386.8 | 11.70 | 45.1 | 31.9 | NoSpike | 15.3 | 250 | 0 | 50 | 0 | 1 | NA | NA | NA |
| 39 | 9 | C5 | 0.432 | 30.3 | 2590.0 | 2693.0 | 5.46 | 147.0 | 104.0 | YesSpike | 19.2 | 250 | 1 | 50 | 25 | 1 | NA | NA | NA |
# Creating variables in indicated units
# dilution (buffer)
df$Buffer_ml <- c(df$Buffer_nl/1000)
# remove unnecessary information
data <- df %>%
filter(CV.Perc < 15) %>%
filter(Binding.Perc < 80 & Binding.Perc > 20) %>%
dplyr::select(Wells, Sample, Category, Binding.Perc, Ave_Conc_pg.ml, Weight_mg, Buffer_ml, Spike, SpikeVol_uL, Dilution, TotalVol_well_uL, Failed_samples)
kable(tail(data, 10))
| Wells | Sample | Category | Binding.Perc | Ave_Conc_pg.ml | Weight_mg | Buffer_ml | Spike | SpikeVol_uL | Dilution | TotalVol_well_uL | Failed_samples | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 21 | B11 | 33 | NoSpike | 52.3 | 1100.0 | 33.8 | 0.25 | 0 | 0 | 1 | 50 | NA |
| 22 | C11 | 34 | NoSpike | 51.7 | 1124.0 | 35.5 | 0.25 | 0 | 0 | 1 | 50 | NA |
| 23 | E11 | 36 | NoSpike | 53.2 | 1062.0 | 31.2 | 0.25 | 0 | 0 | 1 | 50 | NA |
| 24 | F11 | 37 | NoSpike | 36.1 | 2076.0 | 30.8 | 0.06 | 0 | 0 | 1 | 50 | NA |
| 25 | G11 | 38 | NoSpike | 32.5 | 2444.0 | 34.7 | 0.06 | 0 | 0 | 1 | 50 | NA |
| 26 | G3 | 5 | NoSpike | 72.1 | 501.4 | 14.4 | 0.06 | 0 | 0 | 1 | 50 | NA |
| 27 | H3 | 6 | YesSpike | 34.7 | 2204.0 | 13.7 | 0.06 | 1 | 25 | 1 | 50 | NA |
| 28 | A5 | 7 | YesSpike | 30.5 | 2669.0 | 16.4 | 0.06 | 1 | 25 | 1 | 50 | NA |
| 29 | B5 | 8 | NoSpike | 77.8 | 386.8 | 15.3 | 0.25 | 0 | 0 | 1 | 50 | NA |
| 30 | C5 | 9 | YesSpike | 30.3 | 2693.0 | 19.2 | 0.25 | 1 | 25 | 1 | 50 | NA |
dim(data)
[1] 30 12
# remove duplicates
data.og <- data[!is.na(data$Binding.Perc), ]
Formula:
((A/B) * (C/D) * E * 10,000) = F
##################################
##### Calculate final values #####
##################################
# Transform to μg/dl from assay output
data$Ave_Conc_ug.dL <- c(data$Ave_Conc_pg.ml/10000)
data$Final_conc_pg.mg <- c(
((data$Ave_Conc_ug.dL) / data$Weight_mg) * # A/B *
extraction * # C/D *
data$Buffer_ml * 10000 ) # E * 10000
data <- data[order(data$Sample),]
write.csv(data, file.path(data_path, "Data_cort_values_methodA.csv"), row.names = F)
# summary for all samples
summary(data$Final_conc_pg.mg)
Min. 1st Qu. Median Mean 3rd Qu. Max.
2.716 7.820 10.531 16.376 15.336 58.971
kable(tail(data, 7))
| Wells | Sample | Category | Binding.Perc | Ave_Conc_pg.ml | Weight_mg | Buffer_ml | Spike | SpikeVol_uL | Dilution | TotalVol_well_uL | Failed_samples | Ave_Conc_ug.dL | Final_conc_pg.mg | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 24 | F11 | 37 | NoSpike | 36.1 | 2076.0 | 30.8 | 0.06 | 0 | 0 | 1 | 50 | NA | 0.20760 | 5.257403 |
| 25 | G11 | 38 | NoSpike | 32.5 | 2444.0 | 34.7 | 0.06 | 0 | 0 | 1 | 50 | NA | 0.24440 | 5.493718 |
| 26 | G3 | 5 | NoSpike | 72.1 | 501.4 | 14.4 | 0.06 | 0 | 0 | 1 | 50 | NA | 0.05014 | 2.715917 |
| 27 | H3 | 6 | YesSpike | 34.7 | 2204.0 | 13.7 | 0.06 | 1 | 25 | 1 | 50 | NA | 0.22040 | 12.548321 |
| 28 | A5 | 7 | YesSpike | 30.5 | 2669.0 | 16.4 | 0.06 | 1 | 25 | 1 | 50 | NA | 0.26690 | 12.694024 |
| 29 | B5 | 8 | NoSpike | 77.8 | 386.8 | 15.3 | 0.25 | 0 | 0 | 1 | 50 | NA | 0.03868 | 8.216340 |
| 30 | C5 | 9 | YesSpike | 30.3 | 2693.0 | 19.2 | 0.25 | 1 | 25 | 1 | 50 | NA | 0.26930 | 45.584635 |
dim(data)
[1] 30 14
We followed the procedure described in Nist et al. 2020:
“Thus, after pipetting 25μL of standards and samples into the appropriate wells of the 96-well assay plate, we added 25μL of the 0.333ug/dL standard to all samples, resulting in a 1:2 dilution of samples. The remainder of the manufacturer’s protocol was unchanged. We analyzed the assay plate in a Powerwave plate reader (BioTek, Winooski, VT) at 450nm and subtracted background values from all assay wells. In the calculations, we subtracted the 0.333ug/dL standard reading from the sample readings. Samples that resulted in a negative number were considered nondetectable. We converted cortisol levels from ug/dL, as measured by the assay, to pg/mg—based on the mass of hair collected and analyzed using the following formula:
A/B * C/D * E * 10,000 * 2 = F
where - A = μg/dl from assay output; - B = weight (in mg) of collected hair; - C = vol. (in ml) of methanol added to the powdered hair; - D = vol. (in ml) of methanol recovered from the extract and subsequently dried down; - E = vol. (in ml) of assay buffer used to reconstitute the dried extract; 10,000 accounts for changes in metrics; 2 accounts for the dilution factor after addition of the spike; and - F = final value of hair cortisol concentration in pg/mg”
dSpike <- data
##################################
##### Calculate final values #####
##################################
dSpike$Final_conc_pg.mg <-
ifelse(
dSpike$Spike == 1, ## Only spiked samples
((dSpike$Ave_Conc_ug.dL - (std.r)) / # (A-spike) / B
dSpike$Weight_mg)
* extraction * # C / D
dSpike$Buffer_ml * 10000 * 2, # E * 10000 * 2
dSpike$Final_conc_pg.mg
)
write.csv(dSpike, file.path(data_path, "Data_cort_values_methodB.csv"), row.names = F)
# summary for all samples
summary(dSpike$Final_conc_pg.mg)
Min. 1st Qu. Median Mean 3rd Qu. Max.
-29.933 -9.370 4.328 -0.182 9.944 11.763
dSpikeSub <- data[c(data$Spike == 0), ]
summary(dSpikeSub$Final_conc_pg.mg)
Min. 1st Qu. Median Mean 3rd Qu. Max.
2.716 5.087 8.874 7.908 10.486 11.763
kable(tail(dSpike, 10))
| Wells | Sample | Category | Binding.Perc | Ave_Conc_pg.ml | Weight_mg | Buffer_ml | Spike | SpikeVol_uL | Dilution | TotalVol_well_uL | Failed_samples | Ave_Conc_ug.dL | Final_conc_pg.mg | |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 21 | B11 | 33 | NoSpike | 52.3 | 1100.0 | 33.8 | 0.25 | 0 | 0 | 1 | 50 | NA | 0.11000 | 10.576923 |
| 22 | C11 | 34 | NoSpike | 51.7 | 1124.0 | 35.5 | 0.25 | 0 | 0 | 1 | 50 | NA | 0.11240 | 10.290141 |
| 23 | E11 | 36 | NoSpike | 53.2 | 1062.0 | 31.2 | 0.25 | 0 | 0 | 1 | 50 | NA | 0.10620 | 11.062500 |
| 24 | F11 | 37 | NoSpike | 36.1 | 2076.0 | 30.8 | 0.06 | 0 | 0 | 1 | 50 | NA | 0.20760 | 5.257403 |
| 25 | G11 | 38 | NoSpike | 32.5 | 2444.0 | 34.7 | 0.06 | 0 | 0 | 1 | 50 | NA | 0.24440 | 5.493718 |
| 26 | G3 | 5 | NoSpike | 72.1 | 501.4 | 14.4 | 0.06 | 0 | 0 | 1 | 50 | NA | 0.05014 | 2.715917 |
| 27 | H3 | 6 | YesSpike | 34.7 | 2204.0 | 13.7 | 0.06 | 1 | 25 | 1 | 50 | NA | 0.22040 | -10.652409 |
| 28 | A5 | 7 | YesSpike | 30.5 | 2669.0 | 16.4 | 0.06 | 1 | 25 | 1 | 50 | NA | 0.26690 | -4.475488 |
| 29 | B5 | 8 | NoSpike | 77.8 | 386.8 | 15.3 | 0.25 | 0 | 0 | 1 | 50 | NA | 0.03868 | 8.216340 |
| 30 | C5 | 9 | YesSpike | 30.3 | 2693.0 | 19.2 | 0.25 | 1 | 25 | 1 | 50 | NA | 0.26930 | -15.115885 |
##################################
##### Calculate final values #####
##################################
datac <- data
datac$Final_conc_pg.mg <- c(
(datac$Ave_Conc_pg.ml / datac$Weight_mg) * # A/B *
extraction * # C/D *
datac$Buffer_ml) # E
datac <- datac[order(datac$Sample),]
write.csv(datac, file.path(data_path, "Data_cort_values_methodC.csv"), row.names = F)
# summary for all samples
summary(datac$Final_conc_pg.mg)
Min. 1st Qu. Median Mean 3rd Qu. Max.
2.716 7.820 10.531 16.376 15.336 58.971
Spike contribution (pg/mL) = (Vol. spike (mL) x Conc. spike (pg/mL) ) / Vol. reconstitution (mL) or total vol. in well (50uL) (depending on where the spike was added)
# Calculate contribution of spike according to the different volumes in which it was added
# Consider that contribution of spike in serial dilutions gets smaller
# Vol. of spike transformed to mL
data$SpikeVol_ml <- data$SpikeVol_uL/1000
# Concentration of the spike:
std
[1] 3139.5
# Vol. reconstitution (mL) is the total volume in tube or well (sample + spike), after adding spike.
# transform to mL
data$TotalVol_well_mL <- data$TotalVol_well_uL/1000
##( Spike vol. x Spike Conc.)
## ------------------------ / dilution = Spike contribution
## Total vol.
# Cortisol added by spike in wells: 0.0025 mL x 3200 pg/mL = 80 pg
# Calculate cort contribution of spike to each sample
data$Spike.cont_pg.mL <- (((data$SpikeVol_ml * std ) / # Volume of spike * Spike concentration
data$TotalVol_well_mL) / # divided by the total volume (spike + sample)
data$Dilution) # resulting number changes depending on the dilution
dSpiked <- data
##################################
##### Calculate final values #####
##################################
dSpiked$Final_conc_pg.mg <-
ifelse(
dSpiked$Spike == 1, ## Only spiked samples
((dSpiked$Ave_Conc_pg.ml - dSpiked$Spike.cont_pg.mL) / # (A - spike) / B
dSpiked$Weight_mg)
* extraction * # C / D
dSpiked$Buffer_ml, # E *
dSpiked$Final_conc_pg.mg
)
write.csv(dSpiked, file.path(data_path, "Data_cort_values_methodD.csv"), row.names = F)
# summary for all samples
summary(dSpiked$Final_conc_pg.mg)
Min. 1st Qu. Median Mean 3rd Qu. Max.
2.716 5.235 9.806 9.329 11.212 22.002
kable(tail(dSpiked[!is.na(dSpiked$Final_conc_pg.mg) , c("Sample", "Final_conc_pg.mg", "Ave_Conc_pg.ml", "Spike.cont_pg.mL", "Binding.Perc", "Weight_mg", "Buffer_ml", "SpikeVol_uL", "Dilution", "TotalVol_well_uL")],20))
| Sample | Final_conc_pg.mg | Ave_Conc_pg.ml | Spike.cont_pg.mL | Binding.Perc | Weight_mg | Buffer_ml | SpikeVol_uL | Dilution | TotalVol_well_uL | |
|---|---|---|---|---|---|---|---|---|---|---|
| 11 | 23 | 10.484553 | 793.6 | 0.00 | 60.9 | 24.6 | 0.25 | 0 | 1 | 50 |
| 12 | 24 | 10.836520 | 680.2 | 0.00 | 64.8 | 20.4 | 0.25 | 0 | 1 | 50 |
| 13 | 26 | 7.688020 | 1991.0 | 0.00 | 37.3 | 20.2 | 0.06 | 0 | 1 | 50 |
| 14 | 27 | 5.030278 | 1393.0 | 0.00 | 46.0 | 21.6 | 0.06 | 0 | 1 | 50 |
| 15 | 28 | 11.763039 | 839.7 | 0.00 | 59.5 | 23.2 | 0.25 | 0 | 1 | 50 |
| 16 | 29 | 4.427836 | 2072.0 | 0.00 | 36.2 | 36.5 | 0.06 | 0 | 1 | 50 |
| 17 | 3 | 5.085954 | 2287.0 | 1569.75 | 33.9 | 11.0 | 0.06 | 25 | 1 | 50 |
| 18 | 30A | 14.474029 | 2888.0 | 1569.75 | 28.8 | 29.6 | 0.25 | 25 | 1 | 50 |
| 19 | 31 | 4.227453 | 1149.0 | 0.00 | 51.2 | 21.2 | 0.06 | 0 | 1 | 50 |
| 20 | 32 | 10.485849 | 1197.0 | 0.00 | 50.0 | 37.1 | 0.25 | 0 | 1 | 50 |
| 21 | 33 | 10.576923 | 1100.0 | 0.00 | 52.3 | 33.8 | 0.25 | 0 | 1 | 50 |
| 22 | 34 | 10.290141 | 1124.0 | 0.00 | 51.7 | 35.5 | 0.25 | 0 | 1 | 50 |
| 23 | 36 | 11.062500 | 1062.0 | 0.00 | 53.2 | 31.2 | 0.25 | 0 | 1 | 50 |
| 24 | 37 | 5.257403 | 2076.0 | 0.00 | 36.1 | 30.8 | 0.06 | 0 | 1 | 50 |
| 25 | 38 | 5.493718 | 2444.0 | 0.00 | 32.5 | 34.7 | 0.06 | 0 | 1 | 50 |
| 26 | 5 | 2.715917 | 501.4 | 0.00 | 72.1 | 14.4 | 0.06 | 0 | 1 | 50 |
| 27 | 6 | 3.611058 | 2204.0 | 1569.75 | 34.7 | 13.7 | 0.06 | 25 | 1 | 50 |
| 28 | 7 | 5.228140 | 2669.0 | 1569.75 | 30.5 | 16.4 | 0.06 | 25 | 1 | 50 |
| 29 | 8 | 8.216340 | 386.8 | 0.00 | 77.8 | 15.3 | 0.25 | 0 | 1 | 50 |
| 30 | 9 | 19.013346 | 2693.0 | 1569.75 | 30.3 | 19.2 | 0.25 | 25 | 1 | 50 |




Since all spiked samples produce negative values, we will continue our analyses using only non-spiked samples.
# non-spiked samples only
data2 <-data[data$Spike == "No", ]
#two datasets, separated by dilution
data2.06 <- data2[data2$Buffer == "60 uL", ]
data2.25 <- data2[data2$Buffer == "250 uL", ]
#### fit models ####
# model Buffer = 0.06
model06 <- lm(Final_conc_pg.mg ~ Weight_mg,
data = data2.06)
r_squared06 <- summary(model06)$r.squared
# model Buffer = 0.25
model25 <- lm(Final_conc_pg.mg ~ Weight_mg,
data = data2.25)
r_squared25 <- summary(model25)$r.squared
# Calculate residuals
residuals06 <- residuals(model06)
residuals25 <- residuals(model25)
# Quantify residuals
# Mean Absolute Error
mae06 <- mean(abs(residuals06))
# Standard Deviation of Residuals
std_dev06 <- sd(residuals06)
# Mean Absolute Error
mae25 <- mean(abs(residuals25))
# Standard Deviation of Residuals
std_dev25 <- sd(residuals25)
# scatterplot
ggplot(data2, aes(y = Final_conc_pg.mg,
x = Weight_mg,
color = Buffer,
fill = Buffer)) +
geom_point(size = 2.5) +
geom_text(label = c(data2$Sample), nudge_y = 0.75, nudge_x = -0.5) +
geom_smooth(method = "lm",
color = "gold3",
se = TRUE,
alpha = 0.1) +
geom_hline(yintercept = mean(data2$Final_conc_pg.mg),
color = "gray80",
linetype = "dashed") +
geom_hline(yintercept = mean(data2.06$Final_conc_pg.mg),
color = "lightblue3",
linetype = "dashed") +
geom_hline(yintercept = mean(data2.25$Final_conc_pg.mg),
color = "lightpink3",
linetype = "dashed") +
theme_minimal() +
xlim(5, max(data2$Weight_mg) + 4) +
ylim(0, max(data2$Final_conc_pg.mg)+4) +
labs(
title = "Final Cort Concentration and Weight
(Non-spiked only)",
y = "Final Concentration (pg/mg)",
x = "Weight (mg)"
) +
theme(
plot.title = element_text(hjust = 0.5, size = 16, face = "bold"),
axis.title = element_text(size = 14),
axis.text = element_text(size = 12)
) +
# Add R^2 annotation
annotate("text", x = max(data2$Weight_mg) * 0.7,
y = min(data2$Final_conc_pg.mg) * 1.5,
label = paste("R² =", round(r_squared06, 3)),
size = 5, color = "black") +
annotate("text", x = max(data2$Weight_mg) * 0.7,
y = max(data2$Final_conc_pg.mg) * 0.84,
label = paste("R² =", round(r_squared25, 3)),
size = 5, color = "black")

The previous figure shows that:
Error using 0.06 mL buffer
Mean Absolute Error (MAE) 0.06 mL: 0.873
Standard Deviation of Residuals 0.06 mL: 1.377
Error using 0.25 mL buffer
Mean Absolute Error (MAE) 0.25 mL: 0.65
Standard Deviation of Residuals 0.25 mL: 0.86
From this we conclude that using a 250 uL dilution provides more consistent results
sessionInfo()
R version 4.4.3 (2025-02-28)
Platform: aarch64-apple-darwin20
Running under: macOS Sequoia 15.4
Matrix products: default
BLAS: /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/lib/libRblas.0.dylib
LAPACK: /Library/Frameworks/R.framework/Versions/4.4-arm64/Resources/lib/libRlapack.dylib; LAPACK version 3.12.0
locale:
[1] en_US.UTF-8/en_US.UTF-8/en_US.UTF-8/C/en_US.UTF-8/en_US.UTF-8
time zone: America/Detroit
tzcode source: internal
attached base packages:
[1] stats graphics grDevices utils datasets methods base
other attached packages:
[1] dplyr_1.1.4 paletteer_1.6.0 broom_1.0.7 ggplot2_3.5.1
[5] knitr_1.49
loaded via a namespace (and not attached):
[1] sass_0.4.9 utf8_1.2.4 generics_0.1.3 tidyr_1.3.1
[5] prismatic_1.1.2 lattice_0.22-6 stringi_1.8.4 digest_0.6.37
[9] magrittr_2.0.3 evaluate_1.0.1 grid_4.4.3 fastmap_1.2.0
[13] Matrix_1.7-2 rprojroot_2.0.4 workflowr_1.7.1 jsonlite_1.8.9
[17] whisker_0.4.1 backports_1.5.0 rematch2_2.1.2 promises_1.3.0
[21] mgcv_1.9-1 purrr_1.0.2 fansi_1.0.6 scales_1.3.0
[25] jquerylib_0.1.4 cli_3.6.3 rlang_1.1.4 splines_4.4.3
[29] munsell_0.5.1 withr_3.0.2 cachem_1.1.0 yaml_2.3.10
[33] tools_4.4.3 colorspace_2.1-1 httpuv_1.6.15 vctrs_0.6.5
[37] R6_2.5.1 lifecycle_1.0.4 git2r_0.35.0 stringr_1.5.1
[41] fs_1.6.5 pkgconfig_2.0.3 pillar_1.9.0 bslib_0.8.0
[45] later_1.3.2 gtable_0.3.6 glue_1.8.0 Rcpp_1.0.13-1
[49] xfun_0.49 tibble_3.2.1 tidyselect_1.2.1 rstudioapi_0.17.1
[53] farver_2.1.2 nlme_3.1-167 htmltools_0.5.8.1 rmarkdown_2.29
[57] labeling_0.4.3 compiler_4.4.3