A study is carried out to assess the potential of hip protectors to reduce femoral neck fractures in elderly nursing home patients. The average age of the patients was 82 years. Over a two-year period 800 patients were recruited and assigned randomly either to the hip protector group or standard care group.
The results:
Hip protector group: 400 patients - 10 of whom had a femoral neck fracture over the two year period Control group: 400 patients - 20 of whom had a femoral neck fracture over the two year period.
What is the absolute risk reduction?
Correct Answer A: Absolute risk reduction = (Control event rate) - (Experimental event rate)
The absolute risk reduction = CER-EER, where:
Control event rate (CER) = (Number who had particular outcome with the control/ (Total number who had the control)
Experimental event rate (EER) = (Number who had particular outcome with the intervention) / (Total number who had the intervention)
CER = 20 / 400 = 1 / 20 = 0.05
EER = 10 / 400 = 1 / 40 = 0.025
ARR = CER - EER = 0.05 - 0.025 = 0.025
Numbers needed to treat and absolute risk reduction:
Numbers needed to treat (NNT) is a measure that indicates how many patients would require an intervention to reduce the expected number of outcomes by one It is calculated by 1/(Absolute risk reduction) and is rounded to the next highest whole number.
Experimental event rate (EER) = (Number who had particular outcome with the intervention) / (Total number who had the intervention).
Control event rate (CER) = (Number who had particular outcome with the control/ (Total number who had the control).
Absolute risk reduction = CER-EER or EER-CER?
The absolute risk reduction (ARR) may be calculated by finding the absolute difference between the control event rate (CER) and the experimental event rate (EER). You will often find both versions of the above listed in different sources. In some ways in doesn't matter which you use as you will end up with the same answer but from a technical point of view:
*This may be more accurately termed absolute benefit increase, rather than absolute risk reduction.
You are reviewing a new study on the benefit of omega-3 fish oils in patients with established ischaemic heart disease.
What does the power of the study equate to?
Correct Answer C: Power = 1 - the probability of a type II error
Significance tests:
A null hypothesis (H0) states that two treatments are equally effective (and is hence negatively phrased). A significance test uses the sample data to assess how likely the null hypothesis is to be correct.
For example:
'there is no difference in the prevalence of colorectal cancer in patients taking low-dose aspirin compared to those who are not' The alternative hypothesis (H1) is the opposite of the null hypothesis, i.e. There is a difference between the two treatments The p value is the probability of obtaining a result by chance at least as extreme as the one that was actually observed, assuming that the null hypothesis is true. It is therefore equal to the chance of making a type I error (see below).
Two types of errors may occur when testing the null hypothesis:
The power of a study is the probability of (correctly) rejecting the null hypothesis when it is false, i.e. the probability of detecting a statistically significant difference:
Each one of the following is associated with hyperkalaemia, except:
Correct Answer B:
Hyperkalaemia:
Plasma potassium levels are regulated by a number of factors including aldosterone, acid-base balance and insulin levels. Metabolic acidosis is associated with hyperkalaemia as hydrogen and potassium ions compete with each other for exchange with sodium ions across cell membranes and in the distal tubule.
ECG changes seen in hyperkalaemia include tall-tented T waves, small P waves, widened QRS leading to a sinusoidal patternand asystole.
Causes of hyperkalaemia:
Foods that are high in potassium:
*Beta-blockers interfere with potassium transport into cells and can potentially cause hyperkalaemia in renal failure patients - remember beta-agonists, e.g. Salbutamol, are sometimes used as emergency treatment.
**Both unfractionated and low-molecular weight heparin can cause hyperkalaemia. This is thought to be caused by inhibition of aldosterone secretion.
A 35-year-old woman is admitted to the Emergency Department following a deliberate overdose of 50 paracetamol tablets around 10 hours ago. On admission she complains of abdominal pain and lethargy. Her prothrombin time is elevated and arterial blood gases show that she is developing a metabolic acidosis. An urgent transfer to the tertiary liver transplant unit is arranged.
What is the main pathological process seen in the hepatocytes of such patients with fulminant hepatitis?
Correct Answer E: In patients such as this one necrosis affects the entire acinus (panacinar necrosis) resulting in liver failure. This is in contrast to the apoptosis seen in patients with mild cases of viral hepatitis, resulting in the possibility of regeneration and recovery of hepatocellular function.
Acute liver failure: Acute liver failure describes the rapid onset of hepatocellular dysfunction leading to a variety of systemic complications.
Causes:
Features*:
*Remember that 'liver function tests' do not always accurately reflect the synthetic function of the liver. This is best assessed by looking at the prothrombin time and albumin level.
A 72-year-old woman presents with polyuria and polydipsia. Investigations reveal the following:
What is the most likely underlying diagnosis?
Correct Answer C: The PTH level in primary hyperparathyroidism may be normal.
Despite a raised calcium level the parathyroid hormone level is inappropriately normal. This points towards a diagnosis of primary hyperparathyroidism and the other causes (such as myeloma) would lead to a suppression of parathyroid hormone.
Primary hyperparathyroidism:
In exams primary hyperparathyroidism is stereo typically seen in elderly females with an unquenchable thirst and an inappropriately normal or raised parathyroid hormone level. It is most commonly due to a solitary adenoma.
Causes of primary hyperparathyroidism:
Features - 'bones, stones, abdominal groans and psychic moans':
Associations:
Investigations:
Treatment:
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