Tom Lewis and Darunee Whiting
In a previous post we talked about how pathology, or diagnostic medicine, can add value to patient pathways. In this post we show how a lack of attention to the unintended consequences of routine screening leads to patient harm. In the next blog, we will talk about how we tackled this through collaborative working between clinicians and the laboratory.
The rate of submission of Mid Stream Urine samples (MSUs) from primary care has showed an inexorable year on year increase (fig.1).
The rate of submission of Mid Stream Urine samples (MSUs) from primary care has showed an inexorable year on year increase (fig.1).
fig1. Number of urine samples received from primary care per 1000 population
Some of this increase may be accounted for by an ageing population, but much of the new demand appears related to the introduction of primary care quality targets for monitoring of chronic kidney disease. Assessment of kidney damage in these patients requires an assessment of the amount of protein excreted in the urine. A quick way of doing this is to use point of care urine dipsticks. There are many urine dipsticks on the market, and some only detect protein and glucose. However, some of these dipsticks provide multiple additional tests - irrelevant in the management of chronic kidney disease, but of possible utility in other situations, such as determining likelihood of urinary tract infection in patients with vague symptoms of this condition (as an aside, clinical assessment performs better than dipstick assessment in the diagnosis of UTI in patients with clear cut symptoms such as dysuria and frequency).
And here we see the rub - the dangers of testing that is not aligned to a clinical question. These patients with chronic kidney disease who are presenting for routine 'check ups' (and so in whom we would not be worrying about infection) often give positive dipstick results for nitrites and leucocytes - classically described as the dipstick signs of infection. We see the same results in other people - in other words, dipsticks for infection have a high sensitivity but a poor specificity. We then bump into the psychological aspects of a 'positive' test on the clinician - it is very difficult to ignore this 'positive' dipstick. So to deal with this discomfort, the urine sample is sent to the laboratory to look for bacterial growth - traditionally seen as the gold standard arbiter of infection.
There is now a second misunderstanding related to the gold standard nature of urine culture (interestingly perpetuated in much of the literature on urine infection). Clinicians are often taught that a growth of 10^8 organisms per litre is diagnostic of infection. The original work by Kass on this subject did not claim this - rather it showed that a pure growth of this magnitude was predictive of reproducibility of this growth in a research setting. It is important to understand that this is most definitely not the same as a diagnosis of infection in a routine clinic setting. The incidence of asymptomatic colonisation of the urinary tract is high - and there is good evidence that this should not be treated with antibiotics, except in well defined populations (eg. pregnant women). In addition, the quality of urine specimens is often poor from routine settings, with samples frequently contaminated with perineal flora. There then may be substantial delay getting these specimens to the laboratory, and specimen stabilisation is never absolute. The consequence of all this is that the performance of urine culture in routine practice is significantly divorced from that seen in the research setting. To give some idea of the magnitude of this problem, all newly pregnant women are asked to send a urine specimen for culture (as this is a group in whom asymptomatic bacteriuria is a very high risk for causing pyelonephritis). Our laboratory requests repeat samples from all pregnant women if they have no symptoms but a pure growth of organisms. We find that this pure growth can only be reproduced in one third of patients. In other words, the specificity of a pure growth of bacteria for diagnosing UTI is very poor. As with all tests, the prior probability of the condition being present is key when trying to interpret a test result, be it the result of a point of care test (eg. dipstick) or a laboratory test (eg. culture).
So what happens to a patient in whom a pure growth of an organism has been reported, but who has no symptoms of infection. Some time ago I looked at 40 patients from primary care with a pure growth of E. coli. 30 of these patients had symptoms of infection (interestingly the culture result had no impact on their management, apart from in 2 patients who had got better on empirical treatment, but had evidence of in vitro resistance to the chosen antibiotic. Both these patients were given a repeat antibiotic script - an incorrect decision and harmful exposure of the patient to further unnecessary antibiotics). 10 patients had no symptoms of infection, but had had samples sent for just the reasons described above (ie. positive dipsticks on routine screening for other conditions). All these patients were prescribed antibiotics. This is clear harm to the patient driven by inappropriate testing. It is also worth considering the effect of abnormal results on the GP. The abnormal result will need to be considered when it arrives back. The choices are to ignore it or to act. Ignoring a test is not trivial, and comes with an opportunity cost - there has to be a conscious decision not to act, or there is a danger that abnormal results performed for good reasons may be erroneously ignored. This is an ever increasing problem with the inexorable rise of testing. And as we have seen the decision to act can be even more problematic - not only does it take time to 'do something', there is a very real risk that this action will lead either to direct harm to the patient (eg. antibiotic prescribing), or more insidiously perpetuate further work in the system ('failure demand') - bad for the patient and bad for the sustainability of healthcare.
In the next post we will consider how tackling this problem collaboratively as a system has improved outcomes, reduced cost and reconnected healthcare professionals with the purpose of their work.
