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Correlation Between Rapid Strip Test and the Quality of Sputum^*

^* From the Department of Pediatrics (Dr. Gal-Oz), the Infectious Diseases Unit (Dr. Kassis), the Microbiology Laboratory (Dr. Shprecher), and the Pediatric Pulmonary Unit (Drs. Beck and Bentur), Department of Pediatrics, Meyer Children’s Hospital, Rambam Medical Center, Haifa, Israel.

Correspondence to: Lea Bentur, MD, Director, Pediatric Pulmonary Unit, Rambam Medical Center, PO Box 9602, Haifa, Israel, 31096; e-mail: l_bentur{at}rambam.health.gov.il

Study objectives: To evaluate the use of a rapid strip test for the quick evaluation of sputum quality.

Design: Prospective, double-blind study. Sputum and saliva samples were collected. Sputum quality was assessed by the presence of polymorphonuclear neutrophils (PMNs) and squamous epithelial cells (SECs) per low-power (microscopic) field (LPF) [x10 objective]. Sputum was defined as follows: (1) informative (ie, > 25 PMNs and < 10 SECs per LPF); (2) semi-informative (ie, > 25 PMNs or < 10 SECs per LPF, but not both); or (3) uninformative (ie, < 25 PMNs and > 10 SECs per LPF). The first two levels were considered to be "sputum" and the third one was considered to be "nonsputum." The quality of the sputum was compared to results obtained using a rapid strip test (Combur-Test; Roche Diagnostics; Basel, Switzerland) for specific gravity (SG), pH, leukocyte esterase (LE) activity, and levels of nitrites, protein, glucose, and erythrocytes. A Kruskal-Wallis test was used to compare the three levels of sputum quality and the rapid strip test. A Mann-Whitney test compared sputum and nonsputum to the rapid strip reagents. Pearson correlation and tests were used to assess correlation. Receiver operating characteristic was used to calculate the best cut-point values, and the sensitivity and specificity of these values were calculated.

Results: Eighty-two samples were included, with 61 samples from hospitalized patients and 21 samples from healthy volunteers. The best predicator of sputum quality was the SG of the reagent. Using an SG threshold definition of > 1.01, the sensitivity was 86.8% and the specificity was 75.9%. The specificity of protein, glucose +1, and LE levels were relatively low. No relationship was found between the results of the reagent strip test for pH, nitrites, and erythrocytes, and the sputum quality.

Conclusion: Using an SG threshold definition of > 1.01, the rapid reagent strip test has been shown to be a sensitive test for the evaluation of sputum quality, which can be useful when facilities for sputum cytology are not available.

Key Words: pneumonia • polymorphonuclear neutrophils • rapid strip test • reagent strip • sputum