Summary of PHTwitJC #2: Prostate-Specific Antigen Concentration at age 60

Participants and followers of #PHTwitJC voted on their preferred topic to discuss and critique, from a selection of case-control papers put forward.  It was intended for this paper to be discussed on 28th August, however due to low participation it was re-run.

On Sunday September 11th #PHTwitJC discussed a paper by Vickers, A et al ((2010) Prostate specific antigen concentration at age 60 and death or metastasis from prostate cancer: case-control study).  The transcript of discussions can be accessed here.

Q1 – This paper uses a case-control study design – was this appropriate for this topic?

Participants agreed that a case-control design was appropraite as the outcome was suitably rare (death from prostate cancer).  Cases and controls were identified from a previous cohort study (otherwise known as a nested case-control study design); it was suggested that using a clearly defined closed cohort strengthened the design.

Q2 – Did recruitment for cases and controls affect selection bias?

It was unclear from the paper how participants were recruited in the original cohort study, therefore it is difficult to ascertain the probability of selection bias.  It was questioned whether a bias was created through the different inclusion criteria for cases and controls (i.e. cases were dead, but controls were all alive). This is likely to cause a sample selection bias within the control group by discounting those who were dead.  Participants discussed what the implications of this bias would be upon the effect size.

Q3 – What confounding factors could be present?  Have these been accounted for?

A number of potential confounding factors (such as socio-economic status (SES), ethnicity, family history and other health conditions) were not accounted for in this paper.

Q4 – Study concluded that “a single measure of PSA at age 60 is associated with a man’s lifetime risk of death from prostate cancer” – are these results valid?

Participants questioned whether the results of this study were clinically useful, for example

“Stats hold up but can you predict when someone needs treating ie at 60 or 80? Would treat at 60 prolong life?” (@TwitttwooSue)

Participants did not feel confident that a single measure can infer lifetime risk of death from prostate cancer, especially as the study does not consider other causes of death (sample selection bias).  It was noted that the authors used different thresholds for different parts of the analysis, which can be considered problematic.

Q5 – What are the PH implications of this study? How might these influence future practice/policy?

It was noted that other RCT evidence have demonstrated that prostate-specitic antigen can identify cancers, but it has not been shown to prolong life (all-casue mortality).  Although the authors stated that the findings should inform screening programmes, it was noted that this was not discussed in the paper.

A query was raised questioning why investment in research for evidence to support prostate cancer screening continues, despite evidence to date suggesting that prostate cancer is not suitable to a screening programme.  @carotomes wondered whether there was political weight attached to the research due to the lack of male screening programmes, however @TwittwooSue pointed out that the male-only screening programme for Abdominal Aortic Aneurysm (AAA) screening is due to be launched soon.

Overall it was felt that this paper provided insufficient evidence to support the use of PSA as an effective screening tool for prostate cancer.  #PHTwitJC participants agreed that this paper shouldn’t influence change in public health policy or practice.

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