Rapid Responses to:
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Rapid Responses: Rapid Responses published:
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ABSOLUTELY
- Graeme M Mackenzie (26 August 2006)
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Evidence based diagnosis – more of the same or some new ideas too?
- Huw Llewelyn (27 August 2006)
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Unnecessary testing
- Josep More (28 August 2006)
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How to act decisively in the absence of certainty.
- BM Hegde (1 September 2006)
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Graeme M Mackenzie, GP Whitehaven CA28 7RG
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"Testing can become almost an end in itself." This is a scourge, especially for GPs whose work is often underminded by inapporpriate requests for more tests. Patients mistake that approach as better medicine when it is often just a way of getting out of a difficult consultation. Of course in out patient clincis this can mean that you may never see the patient again and therefore there are rewards for this approach as you can feel as if you are doing something and there is a chance you will not have to face the problem again. An experienced GP with excellent clincal acumen can conclude and plan without testing. The patient is seen by another doctor who orders lots of tests and the patient thinks the GP has been sloppy. Only time will show the GP to be right but by then the damage to the relationship is often done. It is also important to talk down investigations. With MRis scans in backs, I will often say, "it is unlikely that the scan will show anything of signficance and even if it does it is unlikely we will do much more than we are doing now." Or with tiredness, "most blood tests in tiredness come back as normal and we need to talk about lifestyle and possible psychological reasons.." It comes down to making sure the buck stays with you. Competing interests: None declared |
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Huw Llewelyn, Consultant Physician Department of Endocrinology and Diabetes, Great Western Hospital, Swindon, SN3 3BB
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If “evidence based diagnosis is still in the dark ages”, then so is evidence based treatment – they are inextricably linked. The doctor’s job is to choose the right treatment. If the diagnosis is wrong then the treatment will be wrong. Inaccurate diagnoses will also affect clinical trials. A treatment well may be ‘evidence based’ because it has worked in a published study. However some patients who would have responded might be left out because of diagnostic inaccuracy. Also, some patients who have no prospect of responding can be included incorrectly in a trial [1]. If these non-responders had been left out the result of the trial might have been better. It is therefore important to assess the performance of tests as ‘gold standards’ for use in diagnostic and treatment criteria, as well as their ability to predict other ‘gold standard target results’. If not, published clinical trial evidence may result in patients being deprived of treatment or given it inappropriately in the name of evidence based medicine. Evidence based diagnosis is about convincing others using shared rules of evidence that a diagnosis (and its implications in terms of treatment) should be accepted by others. There are two types of such evidence. There is the evidence gathered from the individual e.g. “this little girl is aged 2 years 6 months and is 85 cm tall” [2] and facts gathered from groups of patients e.g. “in this survey 2/98 little girls aged 2 years 6 months were 90 cm tall or less”. Evidence based diagnosis means specifying the individual’s facts in addition to pointing to facts relating to that diagnosis in the literature [3]. Bayes Theorem uses ‘unconditional’ initial prior probabilities. This means that it cannot be used to interpret diagnostic leads e.g ‘short stature’ or hyperpyrexia. Diagnostic leads are based on ‘conditional’ probabilities and are used to initiate diagnostic thought processes [3]. However, there are closely related theorems which can be used to interpret diagnostic leads [4] as well as the likelihood ratios also used in Bayes Theorem. These other theorems allow doctors to reason with diagnostic evidence in a more familiar way [3] thus reducing misunderstandings which appear to be common [5]. So to improve evidence based diagnosis we also need to collect better data on diagnostic leads, e.g. short stature, abnormal liver function tests in children, hyperpyrexia, etc. The published evidence that will be given for a diagnosis and any related actions cannot realistically be assembled when actually seeing a patient. A draft evidence-based rationale might be prepared in advance and kept in a personal or published handbook or easily useable computer. It would have to be capable of being accessed within seconds to provide evidence in support of a suspected diagnosis and decision arrived at by using kindness, imagination and commonsense. It could be put into context by inserting the patient’s details into the draft evidence summoned up from a computer [6]. If we are to make progress and allow evidence based diagnosis to emerge from the “dark ages” then in addition to doing more of the same, we may also have to be receptive to new ideas. References: 1. Llewelyn D E H, Garcia-Puig J. How different urinary albumin excretion rates can predict progression to nephropathy and the effect of treatment in hypertensive diabetics. JRAAS 2004, 5; 141-5. 2. Dunkelberg S. A patient's journey: our special girl. BMJ 2006 333: 430-431. 3. Llewelyn H, Ang H, Lewis K, Al-Abdullah A. The Oxford Handbook of Clinical Diagnosis, Oxford: Oxford University Press, 2006. 4. Llewelyn D E H. Assessing the validity of diagnostic tests and clinical decisions. MD thesis, University of London, 1988. 5. Bianchi M T, Alexander B M. Evidence based diagnosis: does the language reflect the theory? BMJ 2006; 333: 442-445. 6. Llewelyn D E H; Ewins D L; Horn J, Evans T G R; McGregor A M. Computerised updating of clinical summaries: new opportunities for clinical practice and research? BMJ 1988, 297, 1504-1506. Competing interests: None declared |
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Josep More, Retired Retired
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I have extensive experience with superfluous testing, both as a patient; and as a hospital based physician, where against my judgement I had to order tests as part of the hospital's policy routine. Yet one specific example is foremost in my mind. A man was admitted to my psychiatric unit, and was found to be significantly anemic. As we could not find any cause for this, we sent him to the teaching hospital for a workup. He returned with the diagnosis of "anemia secondary to extensive venipuncture". He had had a previous admission to that same hospital, for some other diagnostic workup, which had resulted in no significant finding. In the academic zeal for blood tests, he was bled to the point of anemia. Competing interests: None declared |
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BM Hegde, Retired Vice Chancellor Mangalore 575004, India
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Dear Editor, David Eddy, the former cardiovascular surgeon at Stanford turned Duke University mathematics PhD, who has devised a new computer model called ARCHIMEDES, has elegantly shown in retrospect that most, if not all, treatments for chronic diseases like diabetes, high blood pressure and raised cholesterol might not do much good; might even do harm. Herman and colleagues showed in a prospective study of IGT that it is better and much cheaper to change patients’ life style than treat them with drugs as per the evidence based medicine protocol to postpone the onset of diabetes.(1) Delay in onset of diabetes in IGT individuals-(life style change 11 yrs and drugs only 3 years.Reduced incidence(life style 20%, drugs only 8% and, cost (life style $1,100 and drugs $ 31,000).That much for the gold standard of evidence based therapeutics. Now comes the plea for Bayesian theorem in diagnostic methods which are said to be in the dark ages.(2) Having spent nearly half a century on the patient’s bedside trying to make diagnoses, to me this looks like a child playing with esoteric mathematical tools on the beach sands while the large ocean of hidden imponderables in diagnosis lie in the dark unfathomed caves of the ocean. There are very few, if any, black and white regions in the field of diagnoses, most of it falling into the grey zone. One would shudder to think of applying the Bayes’s theorem in the emergency room to make a diagnosis. Even with a ready made computer soft ware it might be too late in the day by the time we calculate all the probabilities for all signs and symptoms before we arrive at evidence based diagnosis. Then there is the enigma called the mind of the patient in the overall picture. Look at what the mind can do without the mathematical jugglery. Harvard Medical School associate professor of medicine Dr. Roger J. Laham reported on follow-up results of a randomized trial looking at laser surgery to improve blood flow. Patients who got the surgery had significantly less pain and improved heart function. But so did patients who had a sham operation -- the equivalent of a placebo. After 30 months the placebo effect was still there. Scans and other tests showed physiological gains in blood flow among only those who thought they had been operated on. This is where the patient’s mind works better than the procedure. The Reverend Thomas Bayes is by far the most enigmatic figure in mathematical history. Almost nothing is known of Bayes's life, and very few of his manuscripts survived. The exact date of Thomas Bayes's birth is not known for certain. In 1742 Bayes was elected a Fellow of the Royal Society of London, the most prestigious scientific body of its day, despite Bayes having published no scientific or mathematical works at that time. ”Bayes's sole publication during his known lifetime was allegedly a mystical book entitled Divine Benevolence, laying forth the original causation and ultimate purpose of the universe. Most mysterious of all, Bayes' Theorem itself appears in a Bayes manuscript presented to the Royal Society of London in 1764, three years after Bayes's supposed death in 1761!” Despite the shocking circumstances of its presentation, Bayes' Theorem was soon forgotten, to be revived later by the efforts of the great mathematician Pierre-Simon Laplace. Laplace himself is almost as enigmatic as Bayes. Laplace's papers are said to have contained a design for a model capable of predicting all future events, the so-called "Laplacian superintelligence". Unusual effects in experiments even today are attributed to a "Laplacian Operator" intervening. ”Bayes' Theorem describes what makes something "evidence" and how much evidence it is. Statistical models are judged by comparison to the Bayesian method because, in statistics, the Bayesian method is as good as it gets - the Bayesian method defines the maximum amount of mileage you can get out of a given piece of evidence, in the same way that thermodynamics defines the maximum amount of work you can get out of a temperature differential.” Previously, the most popular philosophy of science was probably Karl Popper's falsificationism - this is the old philosophy that the Bayesian revolution is trying to currently dethrone. Karl Popper's idea that theories can be definitely falsified, but never definitely confirmed, is another special case of the Bayesian rules. “So there's a limit on how much mileage you can get from successful predictions (3); there's a limit on how high the likelihood ratio goes for confirmatory evidence.” Let us not make doctors’ life more miserable on the bed side. Yours ever, bmhegde References: 1) Herman WH, Hoerger TJ, Brandle M et. al. Cost effectiveness of life style modification and Metformin treatment in preventing Type II diabetes in people with impaired glucose tolerance. Ann. Intern. Med 2005; 142: 323 -332, 2) Matt T Bianchi and Brian M Alexander Evidence based diagnosis: does the language reflect the theory? BMJ 2006 333: 442-445. 3) Smith R. The Screening Industry. BMJ 2003; 326: 889-890 Competing interests: None declared |
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