Audit & Research

Audit Results

We have received relevant karyotype data on all high risk pregnancies for the patients who were due to deliver by June 2007 which means that all the data for 2006 is complete. The expected number of DS cases for the whole period was estimated to be 58 from the fetal medicine first trimester audit package. We have been notified of 69 cases in the same time period.

The average age for the 4 years from 2003 to 2006 was 34.6, 34.6, 34.75 and 34.65 respectively with an overall average age of 34.68 years.

Table 1. is a complex presentation of all the basic data. However, at a glance it contains all the necessary data for the remainder of the report. Table 1. shows the distribution of the number of screening tests performed for a given age or group with the high risk or screen positive results as well as the cases of DS detected and not detected by the screening program using the defined cut-offs. This table is used as a the basis of all subsequent tables within the report.

Table 1. Numbers of patients who had a screen positive or high risk result for DS and the total number of patients screened according to age over the year period as shown . In brackets are the cases of DS – the first number is for those correctly screened for with the second number in the brackets after the comma being the cases of DS not detected by screening. TR screening was introduced in latter part of 2006. Table 2 . Age banding and % chance of having a high risk test result for the year groups of 2003 to 2005 and 2006 respectively
Tricuspid regurgitation ascertainment was introduced in later 2006
Table 2 demonstrates a reduction in screen positive rate across the whole of the age bands within the year groups as demonstrated above.

Table 3. High risk cases screened for DS stratified for year and actual cases of DS recorded to date
In brackets are the percentages of the total for the band in each year
Table 3 shows the number of high risk cases ie 1 in 250 or worse or ‘screen positive for Downs syndrome’ with the number of Downs syndrome cases detected.

Of the 12 cases of DS occurring within the low risk screened group, three of these patients had markers on a second trimester anomaly scan and the pregnancy was terminated. Of the remaining 9 babies born with DS, 1 was from a twin pregnancy with a normal twin but no serum biochemistry and the remaining 8 were singleton. The biochemical risk for the 8 was low risk in 3 of the cases whilst the scan risk was low in all cases. In 2006, 3 of the cases had TR assessed as normal.

Table 4. Two by two tables for each year of audit
HR = high risk test result, LR = low risk test result
The cut off value to determine high risk status or screen test positive was 1 in 250. This was originally used at the time of the scan but changed to a term calculation in May 2006.

Table 5. Screening test performance for each year
Table 6. Outcome of high risk group according to risk status for the period of 2003 to 2006 combined.
In brackets are the percentages of the total for a normal outcome in each risk group
Only one patient had a normal pregnancy outcome in the 1 in 2 risk group. This was where the nasal bone was thought to be absent.

Table 7. Trisomy 21 pregnancies and nasal bone findings
% of total cases in brackets
In the cases of DS and nasal bone data in table 10, the data has been displayed for the years of 2005 onwards as this is when we started using the NB status in the risk calculation. Overall during 2005 to 2006, the NB was present in 53% cases of DS.

In 7 of the cases where the NB was considered present, the prenatal diagnosis of DS was low risk and 5 of the 6 patients went on to have a liveborn DS baby. 1 patient terminated due to markers on a NHS scan and subsequent amniocentesis confirming DS. The NB was considered to be uncertain or not present in 35% of all cases of DS.

Table 8. Outcomes for the fetuses with abnormal TR
There were 25 cases of tricuspid regurgitation (TR) found during 2006 where not all patients were scanned for TF as this was only adopted in the latter part of the year. A total of 1336 patients had an attempt at attaining the TR out of 3176 patients. In the 1336 patients, only twice was the TR recorded as being ‘unable to attain’. This thus gives a TR rate of 1.8% within the population.

Where measured, TR was normal in 2 case of DS and abnormal in 8 other cases of DS. The nasal bone was considered present in 2 cases with an abnormal TR and a pregnancy affected by DS. In these 2 cases the biochemistry was low risk in both with a 4 and 2.8 mm NT measurement, producing a high and low risk respectively. The remaining 6 cases all had absent or uncertain nasal bone presence. However, the risk for NT measurement and biochemistry was already high risk for these cases without the addition of NB data.

Table 9. A list of all cases of trisomy 21 who were screened in the first trimester at MUMS during 2003 to 2006 and classified as low risk.
Of the three cases of DS classified as low risk, 3 terminated before 22 weeks, 2 of these were due to soft markers on scan with a third having a high risk result from a triple test which was inadvertently performed. All three patients had an amniocentesis. Of the remaining 9 patients, 1 patient had ventriculomegaly and echogenic foci on the 20 week anomaly scan but declined any invasive testing as would not consider TOP. Of the remaining 8 patients with DS, none had a major congenital anomaly at birth.

In cases where DS was diagnosed antenatally, only 2 patients elected not to terminate the pregnancy. Both women when questioned were looking for reassurance that the pregnancy was normal . When their high risk status was identified they proceeded to an invasive test to prepare themselves for the birth of a baby with DS but not to proceed with TOP.

Combination of 2006 and 2007 data with TR status known in all patients To date, during 2006 and 2007, since we have introduced TR into the screening program, we have correctly classified 21 cases of Downs Syndrome in 3051 patients screened using all of the above aforementioned 6 criteria. The outcome data is incomplete for the use of TR but to date, 2 cases of DS were classified low risk, both with normal tricuspid valves and no congenital heart lesion. This produces a detection rate of 91% for DS.

There were 88 high risk results with no evidence of trisomy 21. The false positive rate since TR has been introduced into the screening program is now 2.8%. The incidence for DS is 7.5 per 1000 patients screened or 1 case of DS every 132 patients. In 15 of the 21 cases of DS, tricuspid regurgitation was present. All the patients with a 1 in 2 risk had a major chromosomal or anatomical abnormality present. In 2007 so far our false positive rate has dropped to 2.2% from 5.4% for the preceding years.

We have been provided with the age banded second trimester screening data from the Regional screening laboratory based at the Birmingham Womens Hospital. Graph 3 demonstrates the difference between the false positive rates for the first and second trimester testing.

Table 10.Risks before and after screening for Downs Syndrome using the 5 methods listed on the accompanying chart as well as the false positive rate for the test
The graphical changes for the false positive rate are shown below with a further graph depicting average risks pre and post test. The latter is based up on the data in table 10.

Graph 1 showing the risk of a false positive result from the screening tests performed for Downs syndrome between 11 and 14 weeks according to maternal age.
As can be seen on the graph above, the second trimester blood test offered on the NHS has nearly 8 times the risk for testing high risk when compared to the scan and blood test performed at MUMS. The detection rate for Downs at MUMS was also higher than the 75% quoted for the NHS blood test alone.