### Description

For the asymptomatic second degree relative of a patient with Huntington's disease, the risk for having Huntington's disease if the intervening relative is asymptomatic can be calculated from population tables.

Determining the risk:

(1) The first degree relative is usually the parent of the second degree relative.

(2) The risk is read from the tables based on the ages of the first and second degree relatives.

(3) If the first degree relative is not alive, then the age used for the table is the age at which he or she died.

Age of Second

Age of First Degree Relative in Years

Degree

20

25

30

35

40

45

20

0.242

0.237

0.228

0.217

0.197

0.170

25

0.235

0.231

0.222

0.210

0.192

0.165

30

0.222

0.218

0.209

0.199

0.181

0.155

35

0.206

0.202

0.194

0.184

0.167

0.143

40

0.181

0.178

0.170

0.161

0.146

0.125

45

0.149

0.146

0.140

0.132

0.119

0.101

50

0.112

0.109

0.105

0.098

0.089

0.075

55

0.085

0.083

0.079

0.074

0.067

0.056

60

0.056

0.054

0.052

0.049

0.043

0.037

65

0.038

0.037

0.035

0.033

0.030

0.025

70

0.016

0.015

0.015

0.014

0.012

0.010

Age of Second

Age of First Degree Relative in Years

Degree

50

55

60

65

70

20

0.136

0.108

0.074

0.052

0.023

25

0.131

0.104

0.072

0.051

0.022

30

0.123

0.098

0.067

0.047

0.020

35

0.113

0.090

0.061

0.043

0.018

40

0.098

0.077

0.053

0.037

0.016

45

0.079

0.062

0.042

0.029

0.013

50

0.058

0.046

0.031

0.021

0.009

55

0.044

0.034

0.023

0.016

0.007

60

0.028

0.022

0.015

0.010

0.004

65

0.019

0.015

0.010

0.007

0.003

70

0.008

0.006

0.004

0.003

0.001

Implementation Notes:

• The first step is to exclude cases where the ages for the first and second degree relatives are not covered in the table (< 20, > 70).

• The second step is to define the 5 year intervals in the table that cover the ages for the first and second degree relatives. For example, a 24 year old second relative is delimited by the risks for 20 and 25 years, while the 48 year old first relative is delimited by the risks for 45 and 50 years.

• Making the simplifying assumption (which may not be perfectly accurate) that the differences in risk across the age interval is linear, the decimal fraction of the second degree relative's age along her/is 5 year interval is calculated (a 24 year old is 0.8 across the interval of 20 to 25), and then the decimal fraction for the first degree relative along his/er 5 year interval is calculated.

• Finally one moves along the risk intervals to reach the approximate risk. A 24 year old has the risk of a 20 year old, less 80% of the risk difference between a 20 and 25 year old. This is done first along one axis of risk, then along the second axis.