Urinary tract infections (UTIs) constitute one of the most common infections often investigated in many clinical settings [
Proper diagnosis of urinary tract infection is important to ensure prompt and accurate treatment of patients. The laboratory diagnosis of UTI hinges on a set of diagnostic assays including urine dipstick, biochemical test, microscopy, Gram staining, and quantitative urine culture. None of these diagnostic methods in itself is deemed sufficient for a sole diagnosis of UTI as their limitations are widely acknowledged [
Pyuria and bacteriuria are the key indicators of UTI. Nitrite and leucocyte esterase markers on the dipstick are used for the detection of pyuria and bacteriuria, respectively. Nitrite testing relies on the ability to convert nitrate to nitrite. Nitrite production is believed to be associated with the members of
The study collected midstream urine samples from patients suspected of UTI, who were directed by the physician to the laboratory for investigation at Cape Coast Teaching Hospital (CCTH), a tertiary healthcare facility in Ghana, from May to June 2019. CCTH is a major referral healthcare center that serves the populace of Cape Coast and its environs within the central region of Ghana. There was no gender or age restrictions on the participants included in the study. Clinical signs and symptoms were not taken into account. However, the study excluded patients who had used antibiotics in the prior week and patients who had used phenazopyridine in the prior 2 days. To avoid/reduce contamination, all the patients were first informed to wash their hands. They were also taught how to collect clean-catch midstream urine. Besides, all the female patients were informed to wash their genitals with a swab soaked in normal saline. Clean-catch midstream urine was collected in a sterile, wide-mouthed plastic capped bottle.
The dipstick test for the presence of nitrite and leucocyte esterase was conducted using Combur10-Test M-strip following the manufacturer’s instructions (Roche, Canada). With reference to the manufacturer’s guide for interpretation, dipstick testing that produced nitrite or leucocyte esterase result greater than trace was taken as positive.
Well-mixed urine samples were cultured on a plate containing approximately 25 ml of CLED agar (Oxoid, England), blood agar (Oxoid, England), and MacConkey agar (Oxoid, England) within 2 hours after collection using a 0.002-ml sterile loop. The plates were then incubated overnight at 37°C under aerobic conditions. Given the significant risks associated with the use of strict cut-offs, the standard agar-based clinical culture value of 105 colony-forming unit (CFU)/mL was used to represent an arbitrary cut-off [
Data were first recorded using Excel spread sheath and transferred onto Stata statistical software version 14.0 for analysis. The central tendency for age was presented as median (interquartile range). Diagnostic yield of nitrite or leucocyte esterase was calculated as the proportion of urine samples that were nitrite-positive or leucocyte esterase-positive, respectively. Two sample pretest was performed to find the difference in percentage yield by indexed test results (positive and negative) with 95% CI. Sensitivities, specificities, positive and negative predictive values, and positive and negative likelihood ratios were calculated for urine samples with 95% CI. Receiver operating characteristics curve (ROC) was used to generate area under curve (AUC) values to estimate the diagnostic accuracy of the indexed dipstick test [
The study was carried out with an approval clearance from Institutional Review Board Secretariat, protocol ID: UCC (UCCIRB/CHAS/2019/70). Additionally, an informed written consent was obtained from the study participants and confidentiality was kept.
Of the 429 urine samples, 65 patients had urine culture <105 cfu/ml, indicating positive urine culture. The remaining 364 showed either no significant growth (NSG) or no bacteria growth (NBG) with a urine culture. The median age of the patients was 39 years (interquartile range = 35). A proportion of 310 (72.3%) were females, whereas 119 (27.7%) were males.
Nitrite-positive urine samples had substantially higher proportion of samples that were urine culture-positive 18/24 (75.0%, 0.532–0.902) compared with nitrite-negative urine samples 47/405 (11.6%, 0.087–0.151), yielding 62.7% (0.457–0.810,
Causative bacteria isolate with nitrite and leucocyte esterase of quantitative urine culture results.
Organism | Nitrite-negative |
Nitrite-positive |
Leucocyte esterase-negative |
Leucocyte esterase-positive |
Total |
---|---|---|---|---|---|
|
26 (55.3) | 15 (83.3) | 15 (57.7) | 26 (66.7) | 41 (63.1) |
|
14 (29.8) | 3 (16.7) | 10 (38.5) | 7 (17.9) | 17 (26.2) |
|
4 (8.5) | 0 (0) | 1 (3.8) | 3 (7.7) | 4 (6.2) |
|
2 (4.3) | 0 (0) | 0 (0) | 2(5.1) | 2 (3.1) |
|
1 (2.1) | 0 (0) | 0 (0) | 1 (2.6) | 1 (1.5) |
Total | 47 (100) | 18 (100) | 26 (100) | 39 (100) | 65 (100) |
Using culture as the gold standard, the results for nitrite alone had a relatively low sensitivity of 27.7 (95% CI = 17.3–40.2). Positive predictive value (PPV) and negative predictive value (NPV) for nitrite alone were found to be 75.0 (95% CI = 53.3–90.2) and 88.4 (95% CI = 84.9–91.3), respectively. On the other hand, the lone performance of leucocyte esterase showed a sensitivity of 60.0 (47.1–72.0), specificity of 73.9 (95% CI = 69.1–78.3), and positive predictive value of 29.1 (95% CI = 21.6–37.7). The combination of nitrite-positive or leucocyte esterase-positive results yielded the highest sensitivity and NPV value of 72.3 (95% CI = 59.8–89.7) and 93.6 (95% CI = 90.1–96.2), respectively. On the other hand, the combination of nitrite-positive and leucocyte esterase-positive results yielded the lowest sensitivity 16.9 (95% CI 8.8–28.3) but the highest specificity value 99.7 (95% CI = 98.5–100) and PPV 91.7 (95% CI = 61.5–99.8). The results indicated a poor performance of dipstick in ruling out likely negative diagnostic culture, producing negative likelihood ratio (−LR) ranging from 0.38 (0.26–0.57) to 0.83 (0.75–0.93). Details on the performance of dipstick strip with urine culture as the gold standard are presented in Table
Diagnostic performance of nitrite and leucocyte results relative to quantitative urine culture.
Culture | Sensitivity (%) (95% CI) | Specificity (%) (95% CI) | PPV (%) (95% CI) | NPV (%) (95% CI) | +LR (95% CI) | −LR (95% CI) |
---|---|---|---|---|---|---|
NIT+ | 27.7 (17.3–40.2) | 98.4 (96.4–99.4) | 75.0 (53.3–90.2) | 88.4 (84.9–91.3) | 16.8 (6.93–40.72) | 0.74 (0.63–0.86) |
LE+ | 60.0 (47.1–72.0) | 73.9 (69.1–78.3) | 29.1 (21.6–37.7) | 91.2 (87.4–94.2) | 2.30 (1.77–2.99) | 0.54 (0.40–0.73) |
NIT+ or LE+ | 72.3 (59.8–89.7) | 72.5 (67.6–77.1) | 32.0 (24.5–40.2) | 93.6 (90.1–96.2) | 2.63 (2.10–3.30) | 0.38 (0.26–0.57) |
NIT+ and LE+ | 16.9 (8.8–28.3) | 99.7 (98.5–100) | 91.7 (61.5–99.8) | 87.1 (83.4–90.1) | 61.6 (8.1–69.04) | 0.83 (0.75–0.93) |
NIT = nitrite, LE = leucocyte esterase, PPV = positive predictive value, NPV = negative predictive value, +LR = positive likelihood ratio, −LR = negative likelihood ratio.
ROC analysis showed that the combination of nitrite-positive or leucocyte esterase-positive was the best indicator of quantitative urine-positive or urine-negative culture, with a corresponding AUC value of 0.7242. This was followed by leucocyte esterase (AUC = 0.665), nitrite (AUC = 0.632), and nitrite-positive and leucocyte esterase-positive (AUC = 0.5832) in that order (Figure
ROC curve for dipstick diagnosis with urine culture as a gold standard. NIT = nitrite, LE = leucocyte esterase.
Of the 24 positive-nitrite results, 18 (4.2%) were true positive, whereas 6 (1.4%) were false positive compared with quantitative culture. Overall, nitrite results agreed with the quantitative culture results at 87.65% (0.222–0.481) with a kappa value of 0.351. Details on the agreement between nitrite and urine culture results are presented in Table
Concordance between nitrite results and quantitative urine culture results.
NIT | Culture | |||||
---|---|---|---|---|---|---|
Positive | Negative | Agreement | Kappa | 95% CI |
| |
Positive | 18 (4.2%) | 6 (1.4%) | 87.65% | 0.351 | 0.22–0.48 |
|
Negative | 47 (10.9%) | 358 (83.4%) |
NIT = nitrite.
Among 134 positive leucocyte esterase results, 39 (9.1%) were true positive, whereas 95 (22.1%) were false positive using quantitative urine culture as the gold standard. An agreement of 71.79% (0.142–0.330) and a kappa value of 0.233 were recorded between leucocyte esterase results and culture results (Table
Concordance between leucocyte esterase results and quantitative urine culture results.
LE | Culture | |||||
---|---|---|---|---|---|---|
Positive | Negative | Agreement | Kappa | 95% CI |
| |
Positive | 39 (9.1%) | 95 (22.1%) | 71.79 | 0.24 | 0.14–0.33 |
|
Negative | 26 (6.1%) | 269 (62.7%) |
LE = leucocyte esterase.
Comparing the results of “nitrite-positive or leucocyte esterase-positive” with quantitative urine culture, a proportion of 47 (10.9%) and 100 (23.3%) were recorded as true positive and false positive, respectively. The combination of nitrite-positive or leucocyte esterase-positive results agreed with quantitative urine culture at 72.49% with a corresponding kappa value of 0.295 (0.206–0.385) (Table
Concordance between “nitrite-positive or leucocyte esterase-positive” results and quantitative urine culture results.
NIT or LE | Culture | |||||
---|---|---|---|---|---|---|
Positive | Negative | Agreement | Kappa | 95% CI |
| |
Positive | 47 (10.9%) | 100 (23.3%) | 72.49 | 0.30 | 0.21–0.39 |
|
Negative | 18 (4.2%) | 264 (61.5%) |
NIT = nitrite, LE = leucocyte esterase.
Notably, there was very low false-positive 1 (0.2%) but high false-negative results 54 (12.6%) for “nitrite-positive and leucocyte esterase-positive” results (Table
Concordance between “nitrite-positive and leucocyte esterase-positive” results and quantitative urine culture results.
NIT and LE | Culture | |||||
---|---|---|---|---|---|---|
Positive | Negative | Agreement | Kappa | 95% CI |
| |
Positive | 11 (2.5%) | 1 (0.2%) | 87.18 | 0.250 | 0.127–0.374 |
|
Negative | 54 (12.6%) | 363 (84.6%) |
NIT = nitrite, LE = leucocyte esterase.
The present study assessed the diagnostic performance of urine dipstick, showing its potentials and limitations using quantitative urine culture as a reference test. Findings from this study showed that nitrite-positive and leucocyte esterase-positive urine samples relatively produce higher yields of urine culture-positive results than nitrite-negative and leucocyte esterase-negative urine samples respectively. This means nitrite-positive and leucocyte esterase-positive urine samples are arbitrarily expected to yield higher positive urine culture results than nitrite-negative and leucocyte esterase-negative samples respectively. However, the question that remains is to what extent can results produced with dipstick be relied upon in the absence of midstream urine culture?
From Table
Conjunctive performance of nitrite and leucocyte esterase appeared relatively more reliable than the separate results from nitrite and leucocyte esterase as per the results of the present study. Evidently, “nitrite-positive or leucocyte-positive” results appeared to be the best index for distinguishing between positive and negative results for quantitative urine culture, which is similar to an earlier report [
The ability of dipstick to predict negative results may be vital to preventing the risk of unnecessary initiation of antibiotic treatment [
Nitrite alone recorded a relatively higher +LR 16.8 (6.93–40.72) which suggests it may be useful in ruling in UTI. Conversely, it has relatively low −LR 0.74 (0.63–0.86) indicating that it may not be a good indicator for ruling out UTI. Leucocyte alone appeared to be poor at both ruling in and ruling out UTI [+LR 2.30 (1.77–2.99), −LR 0.54 (0.40–0.73)]. The combination of “nitrite-positive and leucocyte-positive” results produced the highest +LR [61.6 (8.1–69.04)] suggesting that it may be the most useful index for ruling in UTI infection. This finding accords with a recent systematic review study that targeted children under the age of five years [
Further, the present study investigated the level of agreement between nitrite and leucocyte esterase with urine culture. The agreement levels of nitrite and leucocyte esterase reported by this study were higher than those reported in Thailand [
This present study may have some limitations. First, there is no clearly accepted bacteria load cut-off for UTI in literature as varying values are adopted by different laboratories and institutions ranging from 102 cfu/mL to 106 cfu/mL [
The results from this study suggest that combined results from nitrite and leucocyte esterase produce better diagnostic performances compared with solo nitrite and leucocyte esterase performances. Also, dipstick appeared as a good predictor of negative urine cultures. Nonetheless, relative to quantitative urine culture, results produced by dipstick should not be overly relied upon for diagnosis. Therefore, quantitative urine cultures should be encouraged especially in the primary healthcare settings.
All data generated or analyzed during this study are included in this published article.
The authors declare that they have no conflicts of interest.
ID visualized and conceptualized the study. ID, MDP, VA, TAE, and RE performed the laboratory work. EQ performed all statistical analysis. EQ prepared the manuscript and ID proofread the manuscript. ID and MDP supervised all aspect of the study.