Logistic Regression

Logistic regression Analyses were subsequently carried out in order to assess the cumulative effect of the variables under investigation to the decision to have an invasive, a non-invasive or

no further test at all. Logistic regression, as a multivariate analysis, will adjust the effect of all the factors under consideration and surface the statistically significant factors towards decision making.

a) The role of the psychological factors under investigation in women’s decision to have an invasive test (CVS)

Initially, logistic regression was carried out on the overall sample (N=414) to explore the role of the 11 variables under investigation on the decision to have an invasive test (CVS).

As illustrated in Table 3.10, none of the 11 variables under investigation were shown to have a significant impact, with a 0.05 level, on women’s decision to have an invasive test. However, a tendency is observed in three of the variables and their impact on women’s decision to have a CVS test. More specifically, anxiety levels were negatively associated with the decision outcome (odds ratio=0.60, p=0.08) suggesting that a woman with increased levels of anxiety is less likely to choose to have an invasive test. In addition, negative attitude to medicine (NAM) and attitudes to prenatal testing (Apretest) were both positively associated with the decision outcome leading to a somewhat controversial finding: women with a negative attitude towards medicine are suggested to be more likely to have an invasive test (odds ratio=1.47, p=0.07) whilst on the other hand, women with a positive attitude to prenatal testing are also more likely to have an invasive test (odds ration=2.43, p=0.07).

Table 3.10

Firth Logistic Regression on the role of psychological factors in decision to have a CVS (N=414)

b) The role of the psychological factors under investigation in women’s decision to have NIPT

A regression analysis was also carried out on the overall sample (N=414) to explore the role of the 11 variables under investigation on the decision to have NIPT.

As illustrated in Table 3.11 three factors reached statistical significance. More specifically, women with a negative attitude to doctors (NAD) are more likely to have a non-invasive test (odds ratio=1.10, p=0.05) as are women with an internal locus of control (odds ratio=1.06, p=0.02). On the other hand, women with a negative attitude to medicine (NAM) are less likely to have a non-invasive test (odds ratio=0.88, p=0.01) as this variable was negatively associated with the decision outcome.

Table 3.11

Firth Logistic Regression on the role of psychological factors in decision to have NIPT (N=414)

c) The inclusion of risk status as a predictive variable in the decision to have an invasive test (CVS)

Logistic regression of CVS as the chosen test (0=otherwise) on the risk status for T21 and T13/T18, and the psychological factors (Anxiety, PAD, PAM, etc.) has shown the statistically significant effect of the T21 risk status (p=0.04) on the final decision of pregnant women towards CVS. The results indicate that in the presence of risk for T21, the results regarding risk for T13/T18 do not contribute to the decision of the pregnant woman towards CVS (p=0.37).

Likewise, no psychological factors statistically contribute to the decision towards CVS (Table 3.12).

Table 3.12

Firth Logistic regression on CVS (N=414), including risk for T21 and T13/T18

Nonetheless, even though there is no statistically significant effect of any of the psychological factors on the decision to have an invasive test, a closer look at the odds ratio values (Table 3.12) indicates some tendencies that may be of interest to take into consideration. An odds ratio indicates the odds of an outcome occurring in the presence of a particular event happening as opposed to the odds of it happening in the absence of that event and therefore, even if not statistically significant it provides important information in that it reveals a tendency that may become significant, i.e. in the presence of a bigger sample size and even if not, the size of the OR (>1) may be indicative of factors that may still influence the outcome to some extent. For example, a more positive attitude to doctors (PAD) indicates that the likelihood of a woman having a CVS is increased (odds ratio=1.36). Likewise, but to a slightly lesser degree, a positive attitude to medicine (PAM) seems to also increase the odds of a woman choosing an invasive test (odds ratio=1.17). Interestingly, negative attitudes to medicine (NAM) seem to have a greater effect on the decision to have a CVS by increasing the likelihood of a woman making this choice (odds ratio=1.32).

d) The inclusion of risk status as a predictive variable in the decision to have a non-invasive test (Harmony)

We proceeded with excluding the cases (N=4) where CVS was chosen, and investigated the association of T21 and T13/T18 risk status as well as the psychological factors to the decision towards NIPT (NIPT N=151-36.8% vs. No Test N=259-63.2%). Results of the firth logistic

regression indicate the significance of both T21 results (p<0.0001) and the T13/T18 results (p<0.0001) in the decision to have NIPT (Likelihood=444.16 p<0.0001). The psychological factors remain insignificant to the model

Table 3.13

Firth Logistic regression on NIPT (N=410) including risk for T21 and T13/T18

Nonetheless, despite not reaching statistical significance a closer look at the odds ratio values (Table 3.13) indicates that a more positive attitude towards medicine (PAM) increases a woman’s likelihood to have NIPT (odds ratio=1.21), as does an increased perceived behavioural control (PBD) (odds ratio=1.25).

e) The impact of the psychological factors under investigation on the decision to have NIPT in women who were low risk in T13/T18 but intermediate risk for T21

Further analysis was carried out in order to cross-tabulate patients’ test decision by the T13/T18 and T21 risk status results (Table 3.14). At a LOW risk for T13/T18, we observe that 99 out of 357 (27.7%) patients chose to do NIPT over ‘No test at all’, while only 9 out of 263 (3.4%) patients at LOW risk for T21 chose to do NIPT (Table 3.14). This is probably due to the fact that 95 out of the 99 LOW T13/T18 patients also had an intermediate risk for T21. This indicates the significance of an identified risk for T21 (over the T13/T18 risk) towards the decision to have

Table 3.14

Patients’ decision outcome depending on risk for T13/T18

Due to the above finding (Table 3.14) that the vast majority of women who were considered low risk for T13/T18 but moderate risk for T21 (N=100) chose to have NIPT (N=95) a logistic regression was also carried out in order to account for the contribution of the factors under investigation in this decision and identify any different patterns with the previous groups.

Table 3.15

Patients’ decision outcome depending on risk for T21

None of the variables under investigation reached statistical significance for this group of women. However, a closer look at the odds ratio values indicates some tendencies that may be

of interest for future research. Firstly, it is suggested that a positive attitude to doctors (PAD) as well as a positive attitude to medicine (PAM) increase the likelihood of a woman in this category (low risk for T13/T18 and moderate risk for T21) to have NIPT (odds ratio=1.27 and 1.29 respectively). Likewise, a positive attitude to prenatal testing and a positive attitude to genetic abnormalities also increase the likelihood of a Harmony test in this group (odds ratio=1.21 and 1.29 respectively). Finally, to an even greater extent than the aforementioned, an increased perceived behavioural control (PBC) also increases the likelihood of a woman in this group to have NIPT (odds ratio=1.37).

Table 3.16

Logistic regression of NIPT test decision for LowT13/T18 and Moderate T21 risk (N=100)

f) The role of socio-demographic factors in women’s decision making process

Due to the limited number of participants choosing to have an invasive test (CVS; N=4), the role of socio-demographic factors was only explored in relation to women’s decision to have a non-invasive test (NIPT; N=151). As illustrated in Table 3.17, in the presence of risk for T21 and/or T13/T18 there is no significant effect of any of the socio-demographic factors on women’s decision to have NIPT. In other words, when a woman has some element of risk identified neither the psychological factors under investigation nor the socio-demographic characteristics

Table 3.17

The role of psychological factors, risk status and socio-demographics in women’s decision making process