Selecting pH cut-offs for the safe verification of nasogastric feeding tube placement: a decision analytical modelling approach

Abstract

Objectives: The existing British National Patient Safety Agency (NPSA) safety guideline recommends testing the pH of nasogastric (NG) tube aspirates. Feeding is considered safe if a pH of 5.5 or lower has been observed; otherwise chest X-rays are recommended. Our previous research found that at 5.5, the pH test lacks sensitivity towards oesophageal placements, a major risk identified by feeding experts. The aim of this research is to use a decision analytic modelling approach to systematically assess the safety of the pH test under cut-offs 1-9.

Materials and methods: We mapped out the care pathway according to the existing safety guideline where the pH test is used as a first-line test, followed by chest x-rays. Decision outcomes were scored on a 0-100 scale in terms of safety. Sensitivities and specificities of the pH test at each cut-off were extracted from our previous research. Aggregating outcome scores and probabilities resulted in weighted scores which enabled an analysis of the relative safety of the checking procedure under various pH cut-offs.

Results: The pH test was the safest under cut-off 5 when there was ≥30% of NG tube misplacements. Under cut-off 5, respiratory feeding was excluded; oesophageal feeding was kept to a minimum to balance the need of chest X-rays for patients with a pH higher than 5. Routine chest X-rays were less safe than the pH test while to feed all without safety checks was the most risky.

Discussion: The safety of the current checking procedure is sensitive to the choice of pH cut-offs, the impact of feeding delays, the accuracy of the pH in the oesophagus, as well as the extent of tube misplacements.

Conclusions: The pH test with 5 as the cut-off was the safest overall. It is important to understand the local clinical environment so that appropriate choice of pH cut-offs can be made to maximise safety and to minimise the use of chest X-rays.

Trial registration number: ISRCTN11170249; Pre-results.

Keywords: adult feeding; decision analysis; diagnostics; nasogastric tube; ph monitoring.

Figure 1

Clinical pathway of using pH test to ensure safety in feeding by nasogastric tubes.

Figure 2

Trade-off between the number of unnecessary X-rays and feeding incidents.

Figure 3

Safety of the checking procedure under pH cut-offs 1–9, showing separate contributions made by each decision outcome to the overall weighted safety scores.