QJM Advance Access originally published online on June 12, 2007
QJM 2007 100(7):464-465; doi:10.1093/qjmed/hcm047
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Diagnostic yield of early morning urine samples in the diagnosis of tuberculosis
Sir,Early morning urine (EMU) samples are sent in significant numbers from patients suspected of having tuberculosis (TB), despite evidence that the diagnostic yield is very low.1 We assessed the diagnostic yield of EMUs and calculated the associated workload, by retrospectively analysing data from our institution from January 2000 to June 2004.
All patients with positive urine cultures for Mycobacterium tuberculosis (MTB) were identified from microbiology records, and medical notes were reviewed. The cost of processing each EMU sample was calculated using the Welcan schedule of time-based unit values for quantifying pathology workload.2
During the study period, EMUs were submitted from 481 patients, of whom 65 had a positive TB culture from another site. The specimens came from in-patients (n = 352), chest clinic patients (n = 39), other out-patients (n = 77) and patients seen by general practitioners (n = 13). Of the 481 patients who had urine analysed, 98 were under the care of respiratory physicians, 53 were from urology, and the remaining 330 were from other specialties.
Of 1122 EMUs submitted for analysis, 15 samples (1.3%) obtained from 10 patients were smear- and/or culture-positive for TB. From these 10 patients, 33 EMUs were analysed, of which seven were smear-positive and 15 culture-positive. All seven smear-positive EMUs (from three patients) were also culture-positive. This resulted in a change in management for only one patient, who was found to have genito-urinary TB and persistent sterile pyuria. The site of TB and characteristics of patients with positive EMUs are shown in Table 1.
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Patient 2 (Table 1) presented to the urology department with symptoms suggestive of recurrent cystitis with persistent sterile pyuria, had normal radiological investigations and responded to 6 months of anti-tuberculous therapy. This was the only case where the finding of a positive EMU actually changed management. All nine remaining patients had other site-specific specimens that were smear- or culture-positive for MTB, in addition to culture-positive EMUs, and hence clinical management was not changed by the result.
The cost of the diagnostic procedure was £ 8.10/specimen of urine, thus 1122 samples incurred a total cost of £ 9088.20, excluding nursing time and transport of specimens to the laboratory.
The number of reported cases of TB has been increasing steadily worldwide over the last few years.3 Our institute serves a population with a relatively high proportion of immigrants from countries of high TB endemicity, especially Black African and Asian populations.4 As a result the workload relating to diagnosis of TB in our microbiology laboratory is high. Most patients in our study from whom EMUs were collected were under the care of non-respiratory physicians.
EMU samples have traditionally been one means of obtaining a microbiological diagnosis of TB,5 but the value of this investigation is doubtful when other samples from the site in question are available for testing. Mortier et al. found that of 7200 EMUs analysed, only 65 (0.9%) urine samples obtained from 33 patients (1.2%) were culture-positive for MTB.2 Our data are remarkably similar: of 1122 EMUs analysed, only 15 (1.3%) from 10 patients were positive for MTB, and management was changed in only one. We propose that EMUs should be sent mainly from patients with symptoms of possible genito-urinary TB, where obtaining other site-specific specimens is difficult.
Whipps Cross University Hospital
London
UK
email: simon.quantrill{at}whippsx.nhs.uk
References
1. Mortier E, Pouchot J, Girard L, Boussougant Y, Vinceneux P. The assessment of urine analysis for the diagnosis of tuberculosis. Br Med J (1996) 312:27–8.
2. Welcan UK. Welcan UK workload measurement system for pathology, manual with schedule of unit values. 1992 edition. (1993 supplement):1–8.
3. Corbett EL, Watt CJ, Walker N, et al. The growing burden of tuberculosis: Global trends and interactions with HIV epidemic. Arch Intern Med (2003) 163:1009–21.
4. North East London TB Network. Tuberculosis in Waltham Forest by ethnic group. London TB register, 2002–2004.
5. Bentz RR, Dimcheff DG, Nemiroff MJ, Tsang A, Weg JG. The incidence of urine culture positive for Mycobacterium tuberculosis in a general tuberculosis patient population. Am Rev Respir Dis (1975) 111:647–50.[Web of Science][Medline]
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