Patients with the Fragile X syndrome may show a variable percentage of affected cells, with some investigators reporting patients with 1-2% affected cells. This means that a sufficient number of cells need to be analyzed in order to reliably exclude the diagnosis in a patient.
|
Maximum Frequency of Expression of Fragile X that Can Be Ruled Out by a Negative Cytogenetic Analysis |
||
|---|---|---|
|
Number of Cells Analyzed |
With 95% Certainty |
With 99% Certainty |
|
5 |
45% |
60% |
|
10 |
26% |
37% |
|
15 |
18% |
26% |
|
20 |
14% |
21% |
|
25 |
11% |
17% |
|
30 |
10% |
14% |
|
35 |
8% |
12% |
|
40 |
7% |
11% |
|
45 |
6% |
10% |
|
50 |
6% |
9% |
|
55 |
5% |
8% |
|
60 |
5% |
7% |
|
65 |
5% |
7% |
|
70 |
4% |
6% |
|
75 |
4% |
6% |
|
80 |
4% |
6% |
|
85 |
3% |
5% |
|
90 |
3% |
5% |
|
95 |
3% |
5% |
|
100 |
3% |
5% |
|
150 |
2% |
3% |
|
200 |
1% |
2% |
|
250 |
1% |
2% |
|
300 |
1% |
2% |
|
350 |
< 1% |
1% |
|
400 |
< 1% |
1% |
from Table 2, De Arce (1983)
where:
• This data could be expressed in a log equation rather than a lookup table.
|
Number of Cells Needed for Cytogenetic Analysis to Identify at least 1 Cell Affected by Fragile X |
||
|---|---|---|
|
Percent Fragile X Cells |
With 95% Certainty |
With 99% Certainty |
|
50% |
4 |
7 |
|
40% |
6 |
9 |
|
30% |
8 |
13 |
|
20% |
13 |
21 |
|
10% |
28 |
44 |
|
9% |
32 |
49 |
|
8% |
36 |
55 |
|
7% |
41 |
63 |
|
6% |
48 |
74 |
|
5% |
58 |
89 |
|
4% |
73 |
112 |
|
3% |
98 |
151 |
|
2% |
142 |
227 |
|
1% |
298 |
458 |
from Table 1, De Arce (1983)
|
Number of Cells Needed for Cytogenetic Analysis to Identify at least 2 Cells Affected by Fragile X |
||
|---|---|---|
|
Percent Fragile X Cells |
With 95% Certainty |
With 99% Certainty |
|
50% |
7 |
10 |
|
40% |
9 |
16 |
|
30% |
14 |
20 |
|
20% |
19 |
29 |
|
10% |
45 |
63 |
|
9% |
47 |
70 |
|
8% |
50 |
74 |
|
7% |
61 |
87 |
|
6% |
70 |
99 |
|
5% |
93 |
154 |
|
4% |
103 |
183 |
|
3% |
161 |
195 |
|
2% |
208 |
299 |
|
1% |
413 |
598 |
from Table 3, De Arce (1983)
Interpretation:
• For affected males and females, exclusion at the 3% level with 95% confidence (100 cells) is generally considered appropriate.
• A positive result at levels below 3-4% should be repeated and verified before accepting the results.
• Molecular techniques with DNA amplification are valuable for patients with low level positivity, and the method complements cytogenetic analysis.
Specialty: Genetics
