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The sensitivity of
microscopy for diagnosing low grade nongonococcal urethritis is
poor.
Poor sensitivity and consistency of
microscopy in the diagnosis of low grade non-gonococcal urethritis.
Smith R, Copas AJ, Prince M, George B,
Walker AS, Sadiq ST.
Sexually
Transmitted Infections 2003;79:487-490.
Summary:
Question
How reliable is the diagnosis of nongonococcal urethritis (NGU), and what
is the variation between and within microscopists, when urethral smears
from men are examined for the presence of polymorphonuclear leukocytes?
Design
This study evaluated the accuracy and variation, between and within
microscopists, of the enumeration of polymorphonuclear leukocytes on Gram
stained urethral smear slides examined blindly two times by ten
experienced microscopists at a London GUM clinic.
Participants
Sixty Gram stained urethral smear slides, 20 with less than 5
polymorphonuclear leukocytes per microscopic high power field (p/hpf), 20
with between 5 and 20 p/hpf, and 20 with more than 20 p/hpf, were selected
by the senior microscopist at a London GUM clinic for testing.
Description of Tests and Diagnostic
Standard
Ten microscopists, who were randomly selected from among 40 experienced
staff at the clinic, determined the p/hpf for each slide by averaging the
number observed in three fields that represented the highest concentration
of polymorphs. Scores were recorded as <5 p/hpf (negative for NGU),
5-20 p/hpf (low grade NGU), or >20 p/hpf (high grade NGU) for each
slide. After every slide was read once, the slides were randomly relabeled
and the same microscopists examined and scored the slides again. The
initial grading of the slides by the senior microscopist (SM) and the
grade of each slide that was determined by the majority agreement of the
readings provided two gold standards with which to measure the results of
the 10 microscopists.
Main Outcome Measures
Two outcomes were calculated. The first was the correct diagnosis relative
to each gold standard. The second was the consistency of the reading,
determined by whether the two results obtained for each slide by each
microscopist were in agreement or not.
Main Results
Nine low grade and five high grade slides were removed from analysis
because of loss of cells, and one slide was read twice by only four
microscopists, providing 914 total readings and 454 paired readings for
the analysis. The overall proportion of different grades of slides that
were correctly scored as either negative or NGU, and the proportion of
slides where there was consistency between two readings of the same slide
by the same microscopist, relative to the two gold standards, are shown in
the table. The slide grades assigned by the 10 microscopists were
significantly different from the grades obtained by the SM (p<0.001)
and consensus (p=0.02) standards. The difference in consistency between
slide grades was also statistically significant (p<0.001 by both
standards). The overall variability between microscopists was not
statistically different for either accuracy or consistency. For low grade
urethritis, there was poor accuracy of microscopy and the proportion of
slides correctly diagnosed varied substantially across microscopists
(range=45-95% by SM and 67-100% by consensus standard). In addition, the
proportion of slides for which consistency between readings occurred was
lowest and varied substantially across microscopists (45-100% by SM and
50-100% by consensus standards) for low grade urethritis.
Authors' Conclusions
Due to the very low accuracy and
reproducibility achieved for low grade NGU, the results of the study do
not support the wide use of microscopy for the diagnosis of NGU in low
prevalence settings such as primary care.
Source of funding:
Not given
For correspondence: Tariq Sadiq,
Department of GUM, St George's Hospital Medical School, Blackshaw Road,
London SW17 0RE, UK. E-mail address: s.sadiq@sghms.ac.uk
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