Confirmation of nasogastric tube position by pH testing.
BACKGROUND
In 2004, the Medicines and Healthcare products Regulatory Agency (MHRA) advised that nasogastric (NG) tube position should be confirmed using pH strips or paper. However, gastric pH is raised by the use of H2-blockers and proton-pump inhibitors (PPIs) potentially producing false negative pH tests resulting in delayed feeding. In addition, colorimetric differentiation using pH strips may be more prone to bias and inaccuracy than direct pH measurements largely used to establish the threshold.


METHODS
To quantify this problem a 1 day survey of all the patients requiring NG and nasointestinal (NI) feeding was undertaken, to establish the numbers of patients receiving H2-Blockers or PPIs, with or without a safe swallow and the methods currently being used to confirm tube positioning. A second observational study was performed to establish the accuracy of six pH strips available to NHS trusts against four unlabelled pH solutions.


RESULTS
Forty-two per cent of patients receiving NG feeding were on H2-blockers or PPIs, including 13% who had a safe swallow for acidic drinks that could be subsequently aspirated to confirm position. In the second study 'testers' correctly identified pH's 3, 4, 5 and 6 with Mackery-Nagel 0-6, BDH 0-6 and 0-14 strips but overestimated pH 4 as pH 5 with Johnson 0-11 paper, underestimated pH 6 as pH 5 with Pehanon 0-12 paper and with Litmus classified pH 3-5 as acid (all), but half also classified pH 6 as acid.


CONCLUSIONS
Theoretically 29% of NG tube positions could not be confirmed by pH testing because of the usage of PPIs or H2-blockers and lack of swallow. Some pH strips are either inaccurate or their result misinterpreted by staff. Large surveys and trials of the actual efficacy and accuracy of pH testing are required.
Taylor
S J
SJ
Department of Nutrition and Dietetics, Frenchay Hospital, Bristol, UK. stephen.taylor@nbt.nhs.uk
Clemente
R
R
D003430:Cross-Sectional Studies; D005745:Gastric Acidity Determination / Q000295:instrumentation*; D006635:Histamine H2 Antagonists / Q000008:administration & dosage / Q000009:adverse effects; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007441:Intubation, Gastrointestinal; D054328:Proton Pump Inhibitors; D017494:Proton Pumps / Q000008:administration & dosage / Q000009:adverse effects; D011934:Reagent Strips / Q000592:standards*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
D006635:Histamine H2 Antagonists; D054328:Proton Pump Inhibitors; D017494:Proton Pumps; D011934:Reagent Strips