We had 23 students participating in the survey for chronic toxoplasmosis (i.e. I
ID: 196582 • Letter: W
Question
We had 23 students participating in the survey for chronic toxoplasmosis (i.e. IgG antibody positive). I randomize the sample wells before putting them into the holder.
In the process of blowing the serum out of the capillary tube, it’s possible that I did not get all of the contents into the tube. In those cases, the optical density would be lower than there actual value, but assume that all of the samples below were handled correctly in calculating the prevalence of Toxoplasma in the class.
The calibrator factor (written on the calibrator factor tube) was 0.4
Student Name
Number
Absorbance
Blank
A1
0.099
Negative Control
A2
0.154
Positive Control
A3
1.219
Calibrator Factor
A4
1.617
Student 1
A6
0.075
Student 2
A7
0.216
Student 3
A8
0.267
Student 4
A9
0.15
Student 5
A10
1.021
Student 6
A11
0.192
Student 7
A12
0.251
Student 8
B1
0.2
Student 9
B2
0.183
Student 10
B3
0.188
Student 11
B4
0.222
Student 12
B5
0.629
Student 13
B6
0.131
Student 14
B7
0.104
Student 15
B8
0.209
Student 16
B9
0.125
Student 17
B10
0.276
Student 18
B11
1.638
Student 19
B12
0.16
Student 20
C1
0.166
Student 21
C2
0.096
Student 22
C3
0.783
Student 23
C4
0.064
Use the above data to calculate the prevalence of Toxoplasma among the students in the parasitology class. You will first need to determine which samples are positive for Toxoplasma, and then you can calculate the prevalence. Also provide the mean antibody index for the samples. Calculate cut-off value: Calibrator OD x Calibrator Factor (CF). Calculate the Ab (Antibody) Index of each determination by dividing the mean values of each sample by cut-off value.
Student Name
Number
Absorbance
Blank
A1
0.099
Negative Control
A2
0.154
Positive Control
A3
1.219
Calibrator Factor
A4
1.617
Student 1
A6
0.075
Student 2
A7
0.216
Student 3
A8
0.267
Student 4
A9
0.15
Student 5
A10
1.021
Student 6
A11
0.192
Student 7
A12
0.251
Student 8
B1
0.2
Student 9
B2
0.183
Student 10
B3
0.188
Student 11
B4
0.222
Student 12
B5
0.629
Student 13
B6
0.131
Student 14
B7
0.104
Student 15
B8
0.209
Student 16
B9
0.125
Student 17
B10
0.276
Student 18
B11
1.638
Student 19
B12
0.16
Student 20
C1
0.166
Student 21
C2
0.096
Student 22
C3
0.783
Student 23
C4
0.064
Explanation / Answer
At first we will subtract Blank OD value from every reading as done in the table below. (This is done to normalise the readings.) These values will now be used further.
Cut-off value = Calibrator OD X Calibrator factor (CF) = 1.518 X 0.4 = 0.6072
Antibody Index will then be calculated for each reading as done in table by dividing the reading of each sample by the cut-off value calculated above (=0.6072).
The antibody index for the negative control is 0.091. Hence all samples with antibody index above 0.091 will be taken as positive for the prevalence of Toxoplasma in the class. It is shown in table by Y (yes) or N (no) for the prevalence of Toxoplasma in each sample.
Now the prevalence of Toxoplasma in the whole class is calculated as below:
Prevalence (in %) =
(Number of students with Toxoplasma (Sample with Y) / Total Number of students sampled) X 100
= (16/23) X 100
69.565%
0
OD OD minus Blank OD Antibody Index Positive (Y/N) Blank 0.0990
Negative Control 0.154 0.055 0.091 Positive Control 1.219 1.12 1.845 Calibrator 1.617 1.518 2.500 Student 1 0.075 -0.024 -0.040 N Student 2 0.216 0.117 0.193 Y Student 3 0.267 0.168 0.277 Y Student 4 0.150 0.051 0.084 N Student 5 1.021 0.922 1.518 Y Student 6 0.192 0.093 0.153 Y Student 7 0.251 0.152 0.25 Y Student 8 0.200 0.101 0.166 Y Student 9 0.183 0.084 0.138 Y Student 10 0.188 0.089 0.147 Y Student 11 0.222 0.123 0.203 Y Student 12 0.629 0.53 0.873 Y Student 13 0.131 0.032 0.053 N Student 14 0.104 0.005 0.008 N Student 15 0.209 0.11 0.181 Y Student 16 0.125 0.026 0.043 N Student 17 0.276 0.177 0.292 Y Student 18 1.638 1.539 2.535 Y Student 19 0.160 0.061 0.100 Y Student 20 0.166 0.067 0.110 Y Student 21 0.096 -0.003 -0.005 N Student 22 0.783 0.684 1.126 Y Student 23 0.064 -0.035 -0.058 NRelated Questions
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