Sunday, August 29, 2010

Classification and probability

Classification is the process of putting people, animals or things into groups or classes according to certain characteristics. So we classify people into children and adults according to age or men and women according to sex. We may subdivide adults into young, middle aged and old according to certain limits of age that we allocate for each group. We may classify diseases according to the organ they primarily involve into e.g. heart diseases, lung diseases, kidney diseases and so on.
Sharing a certain characteristic may indicate sharing other characteristics e.g. old people may be more prone to certain diseases than younger adults or children. These classifications which deal with groups have important effects on our dealing with individuals. In medicine we usually translate the occurrence (incidence or prevalence) of a disease in the group into a probability in the individual. For example, if 50% of old people have osteoporosis, when we have an elderly patient we say he has a 50% chance (probability) of having osteoporosis. If 80% of heart failure in elderly people is caused by atherosclerosis and we have an elderly patient with heart failure we think that the cause of his heart failure is atherosclerosis with a probability of 80% and so on. This translation of occurrence into probability is logical. However it may lead to an erroneous conclusion if done blindly. The following example explains this.
Students are taught that in children nephrotic syndrome is caused by minimal change glomerulonephritis in 80% of cases while only 20% of cases in adults are caused by minimal change disease. The most common cause of nephrotic syndrome in adults, they are taught, is membranous glomerulonephritis. Consequently when they see an 18 or 20 year old man with nephrotic syndrome and you ask them what the most likely cause of his nephrotic syndrome is, many will answer membranous glomerulonephritis because he is an adult. The answer is the effect of classifying adults who represent a very large and heterogeneous collection as one group or class. When adults are taken together membranous glomerulonephritis is the most common cause because the group includes many middle aged and elderly people in whom this disease is the commonest cause. That is not the case in young adults. The 80% likelihood of a minimal change disease being the cause of nephrotic syndrome in a child does not dramatically drop to 20% when the child reaches an age that puts him in the adult category. Nature does not change its behavior according to the limits we use in our classification. The 80% chance in say a 5 year old child may become 75 in a 10 year old, 65 in a 15 year old and 60 in a 20 year old. The probability may drop to 20% in a 40 or 50 year old and to less than that in a 60 or 70 year old man (these imaginary figures are only to explain the idea and are not claimed to be real). In other words minimal change is still the most likely cause of nephrotic syndrome in the very young adult and its probability decreases gradually as the age advances. Classification helps us to understand and remember various scientific facts but we have to keep in mind that the sharp boundaries between classes or subclasses are frequently artificial. They are created by us and not necessarily present in nature.

Friday, August 27, 2010

Relative risk and absolute risk

Statistics can be deceiving if one is not careful. We learn of things that double our risk of developing this or that disease or increase it by say 25 or 50%. We also learn of things that decrease our risk of developing a disease by a certain percentage. It may sound very important to have a risk doubled or reduced to half but unless we know how much it originally was, can we really tell how much the change is? If you get a job and your boss tells you that your salary will be double the salary of Mohammed you immediately ask: How much is the salary of Mohammed? We do not do the same when we learn about doubling or halving the risk of a certain condition.  We do not bother to know how much the original risk was. Unless we translate a relative risk (i.e. a risk expressed as a ratio of another risk) into an absolute risk (i.e. the chance of developing an event regardless of how it compares with another risk) we cannot judge its magnitude and importance. For example, if you know that smoking increases your risk of developing a cardiovascular event in the next 10 years by say 50% and your absolute risk is already 20%, the increase is 10% which is important and worth avoiding smoking (leaving aside other harms of smoking). On the other hand if someone tells you that using your mobile phone increases your chance of developing an acoustic nerve tumour by 100% and your statistical chance of developing this disease (i.e. your absolute risk) is 1/100,000 then using the mobile phone will increase it to 2/100,000 i.e. an increase of 1/100,000. Most people will consider this increase, even if it is true, insufficient to make them stop using mobile phones. I can mention other examples of things that decrease the risk and the same thing applies.
We should always remember that it is the absolute risk that counts.

Monday, August 23, 2010

Our maintenance dialysis policy

Because of the poor dialysis situation in our country (and in similar so called developing countries) we do not usually start our patients on maintenance dialysis early. Most patients have obvious uraemic manifestations when they are started (Creatinine clearance usually 5ml/min or less). Some in fact start their dialysis as an emergency life saving procedure. In contrast most patients with chronic renal failure in developed countries start dialysis as a carefully planned procedure before they develop obvious uraemic manifestations (creatinine clearance of 10 ml/min or more). We have been told that early dialysis improves survival and quality of life.
In a recent randomized controlled trial (called Initiating Dialysis Early and Late –IDEAL) conducted at 32 centers in Australia and New Zealand and reported in the August 12 issue of the New England Journal of Medicine(1); Bruce A. Cooper from Royal North Shore Hospital and Sydney Medical School in Sydney (Australia) and Colleagues examined whether the timing of the initiation of maintenance dialysis influenced survival among patients with chronic kidney disease. Between July 2000 and November 2008, eight hundred twenty eight adults (542 men and 286 women) with progressive chronic kidney disease were randomly assigned to an early start group (creatinine clearance between 10 and 14 ml/min) and a late start group (creatinine clearance between 5 and 7 ml/min or the development of obvious uraemic manifestations). At the end of study the authors concluded that planned early initiation of dialysis in patients with stage V chronic kidney disease was not associated with an improvement in survival or clinical outcomes. In an accompanying editorial, Norbert Lameire and Wim Van Biesen from the University Hospital Ghent in Ghent, Belgium commented : "In our view, the IDEAL trial supports the currently recommended practice, in which most nephrologists start patients on renal-replacement therapy on the basis of clinical factors rather than numerical criteria such as the estimated GFR alone,"
Our dialysis policy may after all not be as bad as we think!!??

(1) Cooper BA, Branley P, Bulfone L, Collins JF, Craig JC, Fraenkel MB, Harris A, Johnson DW, Kesselhut J, Li JJ, Luxton G, Pilmore A, Tiller DJ, Harris DC, Pollock CA, A randomized controlled trial of early versus late initiation of dialysis, N Engl J Med, 2010,363(7):609-19.

Thursday, August 19, 2010

Decision making in our daily medical practice

In any action we take in our daily life we, consciously or not, consider the benefits and harms before deciding to do it. Although we do not make accurate calculations and construct decision trees, we make a rough judgment based on our knowledge and experience. This should certainly be the case when we take any decision concerning our patients. The benefits and harms differ in each individual situation and cannot be generalized. For example, you may put a man with paroxysmal atrial fibrillation who has a prolonged attack every several days or few weeks on long term anticoagulants but you do not do the same with a man who has a short attack every one or two years because you judge that the possible harm of the drug outweighs its benefit. Even if you have two patients with the same frequency and duration of attacks you may put one who is intelligent, educated and has a good laboratory within reach on long term anticoagulants and refrain from doing that to one who is not intelligent, not educated and has no reliable laboratory within reach. You may decide to give a hypotensive drug to a man who is obese, diabetic, heavy smoker, has high cholesterol and has a mild hypertension. In a different way you may be satisfied with an advice of change in diet and life style and follow up in the case of a man with the same level of blood pressure who is slim, not diabetic, non smoker and has normal cholesterol. The trade-off between the benefits of the drug and its side effects is different in the two situations. I can mention innumerable examples concerning various actions like asking for certain investigations, admitting patients to hospital, surgical operations and so on. The writers of medical books and journal articles can only give general guidelines on how best to behave in various situations. They cannot put themselves in your shoes in every possible situation you are likely to meet in your practice. These facts are even more important in the case of doctors practicing in developing countries and guided by books and journals written by people practicing in highly advanced institutions in developed countries.
Do we in our medical practice give enough thought and time for every decision and action we take with every individual patient in the same way we do with other actions we take during our normal daily life??

Friday, August 13, 2010

Science and art in the practice of medicine

Medicine is said to be a mixture of science and art. The question is which part is science and
which part is art?
Science is knowledge based on information obtained through human senses (sometimes aided with various instruments) by observation and experimentation and by logical conclusions derived from this information. Its contents are measurable and reproducible (i.e. gives the same result when repeated by the same or a different person) and it follows strict rules and laws.
Art is a human activity which depends on ill defined characteristics like judgment, intuition, gift and experience. It is not readily measurable, not reproducible and does not usually obey strict rules and laws.

Science in the practice of medicine:
Modern medicine depends to a large extent on various biological sciences like physiology, biochemistry, anatomy, pathology, microbiology, pharmacology etc. It is also increasingly dependent on modern technology which in turn depends on sciences like physics, chemistry and mathematics. The modern doctor, at least the good one, is scientific in his approach to patient's problems. He does not presume that illness is produced by an evil spirit which he should rid the patient off by beating him, or that it is the result of some change in body mixtures for which he has no evidence. Instead he defines the problem, makes a preliminary hypothesis about diagnosis, collects evidence, formulates a main hypothesis, tests it, if valid applies it and if not rejects it and looks for another one. This approach is the same that modern scientists follow when they tackle their problems, the so called "the scientific method".

Art in the practice of medicine:
The first thing a doctor does when he meets a patient is to take the history of the illness. This doctor-patient encounter contains so many things that are not measurable, not reproducible and not controlled by strict rules and laws. The look on the face of the doctor, the tone of his voice, the choice of the questions and the wording of them, the way he listens or interrupts all fall more in the realm of art than science. The doctor tries to give the proper weights for various symptoms but can he measure pain, nausea, or dizziness? Through experience he may attach great importance to some symptoms and trivialize others.
When he feels the abdomen he does not have a measure of the pressure he applies with his hand, and no units with which he accurately measures the tenderness or the consistency of an enlarged liver. If he repeats the examination or some one else does, he may give a different estimate. When he listens to the heart or the chest, he does not usually use a device which measures heart sounds, breath sounds or murmurs in an accurate and reproducible way. The same thing applies to examination of reflexes, mentality, speech, joints, muscles, masses etc.
When he decides to do some tests, has he strict rules which tell him what test should be done and what should not? Or is it a matter of vague ill defined judgment on how much is the test likely to be useful and how much is it likely to be a waste of time and a cause of needless suffering and possible harm?
The same applies when he decides on treatment. Treatment may produce benefit and may do harm. The balance between the two usually depends on that vague thing we call "clinical judgment" which we cannot define, cannot measure and which may vary between different doctors and in the same doctor at different times. It is not measurable and not reproducible.
All these activities fall in the category of art. I can mention a lot more in various fields of medical practice like surgery, obstetrics, pathology etc. and about various procedures which require, beside judgment, manual skills.
This part of medicine, the art of it, is usually learned by experience and by working with someone who is good at it. Reading books and attending lectures is not the way to improve it. This is one of the defects in our medical education system. Students concentrate on the science part of medicine. The art of medicine, which is extremely important in medical practice, is to some extent neglected. We cannot put all the blame on students. Many examiners do not concentrate on the art of medicine when they assess students in clinical examinations. Assessing the art of medicine requires a skill which only experienced examiners have.