Limitation of results

lab scientist

Cytogenetic results

Cytogenetic results from blood samples will be based on the analysis of a minimum of three banded cells.

The presence of mosaicism for any chromosome abnormality is not routinely investigated by the level of analysis performed unless dictated by the clinical referral reason or suggested by an observation during routine analysis.

The standard count for detection of a clone present at greater than 10% level is 30 cells. This is reported in the karyotype comments if carried out.

Prenatal samples

Prenatal reports are based on the analysis of a minimum of three banded cells, and therefore are unlikely to detect mosaicism. It should be noted that the majority of samples sent for prenatal chromosome analysis are taken with a view to screening for uncommon numerical chromosomal abnormalities, particularly Down syndrome.

For technical reasons, the quality of structural analysis on such samples is often conducted at a lower level than that which is required to reliably detect small and unexpected chromosomal deletions and other rearrangements. Although many structural chromosome abnormalities will be detected, those that fall below the limits of resolution of the analysis will be missed.

Oncology samples

For oncology samples a normal cytogenetic result is based on the complete analysis of a minimum of 10 G-banded cells usually with the examination of at least a further 10 cells. The number of cells analysed in abnormal cases or previously abnormal cases may be increased or decreased according to the reason for referral.

A normal in-situ hybridisation result is based on the exclusion of a given abnormality in a minimum of 100 interphase cells. An indication will be given if minimum standards are not reached.

For all cytogenetics analyses the ability to detect subtle, unexpected chromosome rearrangements is governed by the resolution of banding achieved. This is intrinsically highly variable. If the quality of the preparation or extent of the analysis performed is not considered adequate for the reason for referral this will be indicated on the report.

DNA sequencing

Sensitivity of DNA sequencing is over 95%. Since all mutations are checked in two separate amplicons, if possible by two independent methods, sensitivity is >99%.

Rare cases of single nucleotide polymorphisms under the primer binding sites may lead to non-amplification of one allele.

The specificity is 100% where the mutation or type of mutation has been previously reported. Where the change is novel, it may be necessary to carry out family studies and it still may not be possible to reach a conclusion regarding pathogenicity.

Low level mutations

In some circumstances it may prove difficult to detect mutations present at a low level, e.g. in cases of mosaicism or mitochondrial heteroplasmy or where there is a mixed cell population due to malignancy. Sensitivity of detection may be tissue-specific and in some cases alternative sample types may be required.

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