Background: In resource poor settings, measurements for blood pH are often unavailable to health care practitioners. An accurate, quick, and inexpensive method would be useful for measuring blood pH in these settings. Several studies have compared the accuracy of pH paper in pleural fluid samples, but none have compared their accuracy of measuring blood pH. The objective of this project was to verify if nitrazine and phion urine pH strips can accurately measure the pH of human plasma in a hospital setting.
Methods: This was a single center study, convenience sample of patients older than 18 years seen in a high-volume Emergency Department. Nitrazine and phion urine pH strips were tested on centrifuged plasma of 50 patients with a suspected acid-base derangement. Nitrazine samples were recorded in increments of 0.5 (6.5, 7.0, 7.5, 8.0) and phion samples were recorded in increments of 0.25 or 0.5 according to manufacturer’s instruction (6.5, 7.0, 7.25, 7.5, 8.0). This data was then compared against the pH from the Arterial Blood Gas (ABG) or Venous Blood Gas (VBG) samples reported by the hospital laboratory.
Results: 50 patients were enrolled in this study over two years. Three were excluded from data analysis; one for not having a VBG or ABG drawn and two screen failures. Of the included patients, the average age was 57.9 years (SD 14.0) and 51.0% were female. When compared with the ABG or VBG result, the pH paper method was faster on average by 11.3 minutes (SD 12.8). ABG/VBG results were 14 (30%) with acidemia, 24 (51%) with a physiologically normal pH, and 9 (19 %) with alkalemia. The sensitivity and specificity with 95% confidence interval for nitrazine were 46.1% [19.2-74.9%] and 56.7% [37.4–74.5%] for acidemia, and 57.1% [18.4-90.1%] and 44.4% [27.9–61.9%] for alkalemia, respectively. The sensitivity and specificity for phion papers were 78.6% [49.2–95.3%] and 39.4% [22.9–57.9%] for acidemia, and 44.4% [13.7-78.8%] and 68.4% [51.3–82.5%] for alkalemia, respectively. A Pearson correlational linear regression demonstrated a correlation coefficient of 0.166 for nitrazine pH strips and 0.236 for the phion pH strips, indicating both strips had a negligible correlation.
Conclusion: Nitrazine and phion pH strips were neither sensitive, specific, nor accurate enough in detecting acidemia and alkalemia to be used to make clinical decisions.