Posts filed under Risk (133)

October 22, 2014

Screening the elderly

I’ve seen two proposals recently for population screening of older people. They’re probably both not good ideas, but for different reasons.

We had a Stat of the Week nomination for a proposal to screen people over 65 for depression at ordinary GP visits, to prevent suicide. The proposal was based on the fact that 70% of the suicides were in people who had visited a GP within the past month.  If the average person over 65 visits a GP less than about 8.5 times a year, this means those visiting their GP are at higher risk.  However, the risk is still very small: 225 over 5.5 years is 41/year, 70% of that is 29/year.

To identify those 29, it would be necessary to administer the screening question to a lot of people, at least hundreds of thousands. That in itself is costly; more importantly, since the questionnaire will not be perfectly accurate there will be  tens of thousands of positive results. For example, a US randomised trial of depression screening in people over 60 recruited 600 participants from 9000 people screened. In the ‘usual care’ half of the trial there were 3 completed suicides over the next two years; in those receiving more intensive and focused help with depression there were 2. The trial suggests that screening and intensive intervention does help with symptoms of major depression (probably at substantial cost), but it’s not likely to be a feasible intervention to prevent suicide.

 

The other proposal is from the UK, where GPs will be financially rewarded for dementia diagnoses. In contrast to depression, dementia is pretty much untreatable. There’s nothing that modifies the course of the disease, and even the symptomatic treatments are of very marginal benefit.

The rationale for the proposal is that early diagnosis gives patients and their families more time to think about options and strategies. That could be of some benefit, at least in the subset of people with dementia who are able and willing to talk about it, but similar advance planning could be done — and perhaps better — without waiting for a diagnosis.

Diagnosis isn’t like treatment. As a British GP and blogger, Martin Brunet, points out

We are used to being paid for things of course, like asthma reviews and statin prescribing, and we are well aware of the problems this causes – but at least patients can opt out if they don’t like it.

They can refuse to attend a review, decline our offer of a statin or politely take the pill packet and store it unopened in the kitchen cupboard. They cannot opt out of a diagnosis.

 

September 26, 2014

Screening is harder than that

From the Herald

Calcium in the blood could provide an early warning of certain cancers, especially in men, research has shown.

Even slightly raised blood levels of calcium in men was associated with an increased risk of cancer diagnosis within one year.

The discovery, reported in the British Journal of Cancer, raises the prospect of a simple blood test to aid the early detection of cancer in high risk patients.

In fact, from the abstract of the research paper, 3% of people had high blood levels of calcium, and among those,  11.5% of the men developed cancer within a year. That’s really not strong enough prediction to be useful for early detection of cancer. For every thousand men tested you would find three cancer cases, and 27 false positives. What the research paper actually says under “Implications for clinical practice” is

“This study should help GPs investigate hypercalcaemia appropriately.”

That is, if a GP happens to measure blood calcium for some reason and notices that it’s abnormally high, cancer is one explanation worth checking out.

The overstatement is from a Bristol University press release, with the lead

High levels of calcium in blood, a condition known as hypercalcaemia, can be used by GPs as an early indication of certain types of cancer, according to a study by researchers from the universities of Bristol and Exeter.

and later on an explanation of why they are pushing this angle

The research is part of the Discovery Programme which aims to transform the diagnosis of cancer and prevent hundreds of unnecessary deaths each year. In partnership with NHS trusts and six Universities, a group of the UK’s leading researchers into primary care cancer diagnostics are working together in a five year programme.

While the story isn’t the Herald’s fault, using a photo of a man drinking a glass of milk is. The story isn’t about dietary calcium being bad, it’s about changes in the internal regulation of calcium levels in the blood, a completely different issue. Milk has nothing to do with it.

September 10, 2014

Cannabis graduation exaggeration

3News

Teenagers who use cannabis daily are seven times more likely to attempt suicide and 60 percent less likely to complete high school than those who don’t, latest research shows.

Me (via Science Media Center)

“The associations in the paper are summarised by estimated odds ratios comparing non-users to those who used cannabis daily. This can easily be misleading to non-specialists in two ways. Firstly, nearly all the statistical evidence comes from the roughly 1000 participants who used cannabis less than daily, not the roughly 50 daily users — the estimates for daily users are an extrapolation.

“Secondly, odds ratios are hard to interpret.  For example, the odds ratio of 0.37 for high-school graduation could easily be misinterpreted as a 0.37 times lower rate of graduation in very heavy cannabis users. In fact, if the overall graduation rate matched the New Zealand rate of 75%, the rate in very heavy cannabis users would be 53%, and the rate in those who used cannabis more than monthly but less than weekly would be 65%.

That is, the estimated rate of completing high school is not 60% lower, it’s about 20% lower.  This is before you worry  about the extrapolation from moderate to heavy users and the causality question. The 60% figure is unambiguously wrong. It isn’t even what the paper claims.  It’s an easy mistake to make, though the researchers should have done more to prevent it, and that’s why it was part of my comments last week.

You can read all the Science Media Centre commentary here.

 

[Update: The erroneous '60% less likely to complete high school' statement is in the journal press release. That's unprofessional at best.]

(I could also be picky and point out 3News have the journal wrong: The Lancet Psychiatry, which started this year, is not the same as The Lancet, founded in 1823)

August 15, 2014

Cancer statistics done right

I’ve mentioned a number of times that statistics on cancer survival are often unreliable for the conclusion people want to draw, and that you need to look at cancer mortality.  Today’s story in Stuff is about Otago research that does it right:

The report found for 11-year timeframe, cancer-specific death rates decreased in both countries and cancer mortality fell in both countries. But there was no change in the difference between the death rates New Zealand and Australia, which remained remained 10 per cent higher in New Zealand.

That is, they didn’t look at survival after diagnosis, they looked at the rate of deaths. They also looked at the rate of cancer diagnoses

“The higher mortality from all cancers combined cannot be attributed to higher incidence rates, and this suggests that overall patient survival is lower in New Zealand,” Skegg said.

That’s not quite as solid a conclusion — it’s conceivable that New Zealand really has higher incidence, but Australia compensates by over-diagnosing tumours that wouldn’t ever cause a problem — but it would be a stretch to have that happen over all types of cancer combined, as they observed.

 

August 14, 2014

Breast cancer risk and exercise

Stuff has a story from the LA Times about exercise and breast cancer risk.  There’s a new research paper based on a large French population study, where women who ended up having a breast cancer diagnosis were less likely to have exercised regularly for the past five year period.  This is just observational correlation, and although it’s a big study, with 2000 breast cancer cases in over 50000 women, the evidence is not all that strong (the uncertainty range around the 10% risk reduction given in the paper goes from an 18% reduction down to a 1% reduction).  Given that,  I’m a bit unhappy with the strength of the language in the story:

For women past childbearing age, a new study finds that a modest amount of exercise — four hours a week of walking or more intensive physical activity such as cycling for just two hours a week — drives down breast cancer risk by roughly 10 per cent.

There’s a more dramatically wrong numerical issue towards the end of the story, though:

The medications tamoxifen and raloxifene can also drive down the risk of breast cancer in those at higher than average risk. They come with side effects such as an increased risk of deep-vein thrombosis or pulmonary embolism, and their powers of risk reduction are actually pretty modest: If 1000 women took either tamoxifen or raloxifene for five years, eight breast cancers would be prevented.

By comparison, regular physical activity is powerful.

Using relative risk reduction for the (potential) benefits of exercise and absolute risk reduction for the benefits of the drugs is misleading. Using the breast cancer risk assessment tool from the National Cancer Institute, the five-year breast cancer risk for a typical 60 year old is perhaps 2%. That agrees with the study’s 2000 cases in 52000 women followed for at least nine years.  If 1000 women with that level of risk took up regular exercise for five years, and if the benefits were real,  two breast cancers would be prevented.

Exercise is much less powerful than the drugs, but it’s cheap, doesn’t require a doctor’s prescription, and the side-effects on other diseases are beneficial, not harmful.

August 7, 2014

New breast cancer gene

The Herald has a pretty good story about a gene, PALB2, where there are mutations that cause a substantially raised risk of breast cancer.  It’s not as novel as the story implies (the first sentence of the abstract is “Germline loss-of-function mutations in PALB2 are known to confer a predisposition to breast cancer.”), but the quantified increase in risk is new and potentially a useful thing to know.

Genetic testing for BRCA mutations is funded in NZ for people with a sufficiently strong family history, but the policy is to test one of the affected relatives first. This new gene demonstrates why.

If you had a high-risk family history of breast cancer, and tested negative for BRCA1 and BRCA2 mutations, you might assume you had missed out on the bad gene. It’s possible, though, that your family’s risk was due to some other mutation — in PALB2, or in another undiscovered gene — and in that case the negative test didn’t actually tell you anything. By testing a family member  first, you can be sure you are looking in the right place for your risks, rather than just in the place that’s easiest to test.

August 4, 2014

Predicting blood alcohol concentration is tricky

Rasmus Bååth, who is doing a PhD in Cognitive Science, in Sweden, has written a web app that predicts blood alcohol concentrations using reasonably sophisticated equations from the forensic science literature.

The web page gives a picture of the whole BAC curve over time, but requires a lot of detailed inputs. Some of these are things you could know accurately: your height and weight, exactly when you had each drink and what it was. Some of them you have a reasonable idea about: is your stomach empty or full, and therefore is alcohol absorption fast or slow. You also need to specify an alcohol elimination rate, which he says averages 0.018%/hour but could be half or twice that, and you have no real clue.

If you play around with the interactive controls, you can see why the advice given along with the new legal limits is so approximate (as Campbell Live is demonstrating tonight).  Rasmus has all sorts of disclaimers about how you shouldn’t rely on the app, so he’d probably be happier if you don’t do any more than that with it.

July 29, 2014

A treatment for unsubstantiated claims

A couple of months ago, I wrote about a One News story on ‘drinkable sunscreen’.

In New Zealand, it’s very easy to make complaints about ads that violate advertising standards, for example by making unsubstantiated therapeutic claims. Mark Hanna submitted a complaint about the NZ website of the company  selling the stuff.

The decision has been released: the complaint was upheld. Mark gives more description on his blog.

In many countries there is no feasible way for individuals to have this sort of impact. In the USA, for example, it’s almost impossible to do anything about misleading or unsubstantiated health claims, to the extent that summoning a celebrity to be humiliated publicly by a Senate panel may be the best option.

It can at least produce great television: John Oliver’s summary of the Dr Oz event is viciously hilarious

July 27, 2014

Air flight crash risk

David Spiegelhalter, Professor of the Public Understanding of Risk at Cambridge University, has looked at the chance of getting three fatal plane crashes in the same 8-day period, based on the average rate of fatal crashes over the past ten years.  He finds that if you look at all 8-day periods in ten years, three crashes is actually the most likely way for the worst week to turn out.

He does this with maths. It’s easier to do it by computer simulation: arrange the 91 crashes randomly among the 3650 days and count up the worst week. When I do this 10,000 times (which takes seconds). I get

crashes

 

The recent crashes were separate tragedies with independent causes — two different types of accident and one deliberate shooting — they aren’t related like, say, the fires in the first Boeing Dreamliners were. There’s no reason for the recent events should make you more worried about flying.

July 23, 2014

Average and variation

Two graphs from the NZ influenza surveillance weekly update (PDF, via Mark Hanna)

flu-averageflu-varying

Both show that the seasonal epidemic has started.  I think the second graph is more helpful in comparing this year to the past; showing the actual history for a range of years, rather than an average.  This sort of graph could handle a larger number of past years if they were all or mostly in, eg, thin grey lines, perhaps with this year, last year, and the worst recent year in colour.

The other news in the surveillance update is that the flu viruses that have been examined have overwhelming been H1N1 or H3N2, and both these groups are covered in this year’s vaccine.