Practical pH Testing for Nasogastric Tube Verification: A Prospective Method-Comparison Study of Low-Cost Handheld...

Nasogastric (NG) tubes are widely used for enteral nutrition, medication administration, and gastric decompression, but misplacement can cause serious, preventable harm. International guidelines recommend gastric aspirate pH testing with pH strips to reduce reliance on X-rays, but subjective color interpretation and stock shortages limit their reliability in resource-constrained settings. Commercial handheld pH meters offer objective digital readings and reusability, but their performance in routine clinical practice is uncertain.

A prospective method-comparison study was conducted among adult inpatients requiring NG tube insertion in a South African public hospital. Gastric aspirate pH was measured using pH strips, followed immediately by a commercially available handheld pH meter on the same sample. Agreement was assessed using Bland-Altman analysis and Cohen's kappa, and diagnostic accuracy at a threshold of pH ≤ 5.5 was secondarily explored using routinely obtained chest radiography, where available, as the reference standard.

Forty-five patients were enrolled; gastric aspirate was obtained from 40 (89%), and 26 (58%) had both an aspirate and an interpretable chest X-ray. The handheld pH meter showed a mean bias of +0.17 pH units, with 95% limits of agreement from -0.46 to +0.80. At pH ≤ 5.5, categorical agreement with pH strips was substantial (κ = 0.754; p < 0.001). Diagnostic accuracy estimates for the pH meter when using radiology as the standard were limited by the small radiograph sample, but showed a sensitivity of 80.0% (95% CI 56.3-94.3%), a specificity of 100% (54.1-100%), a positive predictive value of 100% (79.4-100%) and a negative predictive value of 60.0% (26.2-87.8%). pH strips performed similarly.

In this cohort, a low-cost handheld pH meter showed a small mean bias and substantial categorical agreement with pH strips at pH ≤ 5.5. Its objective digital readout and reusability address key limitations of color-based strips and support its use in resource-constrained settings as a practical bedside tool for confirming gastric NG tube placement. However, pH values > 5.5 do not reliably exclude gastric placement, and chest radiography remains necessary when results are inconclusive or discordant with clinical expectations.