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Open Access
Research
Selecting pH cut-offs for the safe
verification of nasogastric feeding tube
placement: a decision analytical
modelling approach
Melody Zhifang Ni,1 Jeremy R Huddy,1 Oliver H Priest,1 Sisse Olsen,2
Lawrence D Phillips,3 Patrick M M Bossuyt,4 George B Hanna1
To cite: Ni MZ, Huddy JR,
Priest OH, et al. Selecting pH
cut-offs for the safe verification
of nasogastric feeding tube
placement: a decision analytical
modelling approach. BMJ Open
2017;7:e018128. doi:10.1136/
bmjopen-2017-018128
► Prepublication history and
additional material for this
paper are available online. To
view these files, please visit the
journal (http://dx.doi.org/10.
1136/bmjopen-2017-018128).
Received 8 June 2017
Revised 18 September 2017
Accepted 29 September 2017
1
Department of Surgery and
Cancer, St Mary’s Hospital,
Imperial College London,
London, UK
2
Specialist Surgery, Royal
Devon and Exeter Hospital NHS
Foundation Trust, Exeter, UK
3
Department of Management,
London School of Economics
and Political Sciences, London,
UK
4
Department of Clinical
Epidemiology, Biostatistics and
Bioinformatics (KEBB), Academic
Medical Center, University of
Amsterdam, Amsterdam, The
Netherlands
Correspondence to
Dr Melody Zhifang Ni;
z.ni@imperial.ac.uk
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.
InTRODuCTIOn
Every year at least 1 million nasogastric (NG)
tubes are being used in the UK1 and 1.5 billion
worldwide.i Inadvertent tube placement
i
Worldwide usage of 1.5 billion was estimated from NHS
Strengths and limitation of this study
► A decision analytic approach was used to map
►
►
►
►
out clinical pathways and to achieve synthesis of
evidence from clinical studies, published literature
and expert judgements.
The entire range of pH cut-offs was analysed in
addition to the most frequently used ones between
4 and 6.
Two non-pH test strategies were analysed using the
same framework: routine chest X-rays and feeding
all patients without safety checks.
We did not consider financial costs in this analysis.
The same framework can be expanded to incorporate
additional dimensions of importance.
We focused only on the group of patients with
successful aspirations. Unsuccessful aspiration
does not change the relative safety of various pH
cut-offs but is one reason for using chest X-rays.
Our analysis assumed that chest X-rays were
100% accurate. However, reducing pH cut-offs
will not increase misfeeding due to chest X-ray
misinterpretations.
outside the stomach has been classified as a
‘never event’ by NHS England.ii Nevertheless,
incidents of tube misplacements remained
common place. Reported rates of misplacement on insertion and tube migration after
correct initial placement varied between
1.3% and 50% in adults.2 Misplacement into
the respiratory tract occurs in 1%–3% of
patients3 and can have catastrophic consequences, including death. Guidelines on
nutrition support for adults issued by the
National Institute for Health and Care
Excellence recommend that the position of
NG tubes be verified on initial placement
and before each use.4 The British National
usage by assuming demand proportional to population
size.
ii
h t t p s : / / w w w. e n g l a n d . n h s . u k / w p - c o n t e n t /
uploads/2015/03/never-evnts-list-15-16.pdf
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
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Figure 1 Clinical pathway of using pH test to ensure safety in feeding by nasogastric tubes.
Patient Safety Agency (NPSA) recommends testing the
pH of tube aspirates.5–7 Feeding can only start if a pH at
or below 5.5 has been established; otherwise chest X-rays,
the gold-standard, should be used.
Commissioned by NPSA, we investigated evidence
behind various bedside tests including pH, aspirate
appearance, capnometry/colorimetric, auscultation
(‘whoosh’ test) and magnetic guidance.8 The pH test has
the best bedside usability and accuracy underpinned by
a large body of clinical evidence. In addition to respiratory placements, oesophageal placements emerged as
a major safety concern during our consultations with
feeding experts.9 To address this and to remedy the lack
of published studies in oesophageal pH from NG tubes,
we carried out a literature review of pH distributions in
patients with reflux. We found that reducing the cut-off
from 5.5 to 4 would increase the sensitivity of the pH test
to tubes placed in the oesophagus. Subsequent safety
recommendations continued to uphold 5.5 as the safety
threshold.10 The main disadvantage of lowering the pH
cut-off is that more patients with tubes placed inside the
stomach will be sent for chest X-rays, which is the secondline test. This is not ideal since chest X-rays are not only
more expensive—on average each chest X-ray costs £30
whereas a tube of 100 pH strips costs slightly over £10iii—
but also can delay feeding for up to 47 hours.iv In addition,
chest X-rays, despite being considered the gold-standard
of tube site verifications, are subject to misinterpretation
errors.7 11
A drawback of our previous research was an exclusive
focus on the risks from various bedside tests. However,
the recommended checking procedure, in fact, uses a
combination of two tests: the pH test followed by chest
iii
Costs of chest X-rays were derived from NHS reference price 2015 and
price of pH strips were from NHS supply chain website (https://www.
supplychain.nhs.uk/).
iv
Mean delay 17 hours, range 1.5 hours–47 hours. Unpublished audit
data carried out in 2016 at the St Mary’s hospital, London, UK from 23
patients.
2
X-rays should the pH test fail. The question of selecting
suitable pH cut-offs must be addressed using the same
context. We are primarily interested in understanding the
trade-offs in patient safety between maximising feeding
in time and minimising feeding incidents. The aim of
this research is to employ a decision analytic modelling
approach12 which allows us to systematically analyse the
safety of pH test under various cut-offs when embedded
in the clinical setting13 to better inform policy makers and
clinicians performing safety checks.
MATeRIAlS AnD MeThODS
Analysis of the national Reporting and learning System
To provide an overview of the feeding incidents, we
carried out a narrative analysis of incident reports
submitted to the National Reporting and Learning
System (NRLS). We included all cases with evidence of
nasogastric tube misplacement at any site outside the
stomach between October 2003 and 28 February 2009.
Paediatric cases were excluded. Two authors (OHP and
SO) independently reviewed the adverse event reports
and classified these according to whether or not current
safety guidelines were followed (cut-off 5.5). For reports
containing sufficient details to enable an analysis of
possible reasons of tube misplacements, we extracted
the reported reasons for misfeeding and carried out
thematic analyses to generate categories. Disagreements
between the two reviewers were solved by discussion until
consensus was reached.
Safety of ph under various cut-offs
Study design
We mapped out the clinical pathway of the safety guidelines with regard to NG tube feeding (figure 1). Since
our target was the relative safety between different pH
cut-offs, we focused on the subgroup of patients for whom
aspirations were successful but analysed implications of
unsuccessful aspirations on patient safety in the sensitivity
analysis. We assumed that the pH cut-off values could take
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
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Open Access
any number between 1 and 9 (the recommended range),
as well as 5.5 (the current recommendation). Decision
outcomes were scored with points out of 100, with 100
assigned to the outcomes with the best safety and 0 to the
outcomes with the worst safety. The sensitivities and specificities of the pH test under various cut-offs were derived
from our previous research. Aggregating the outcome
scores by their respective probabilities resulted in a set
of weighted scores. These weighted scores enabled a
comparison of the relative safety of the pH test under
various cut-offs.
Outcomes of feeding decisions
Decision outcomes were identified from the clinical
pathway (figure 1), assuming that all patients have
successful aspirations. There were five outcomes in total.
Feeding into the stomach by pH took place if the pH was at
or below a certain cut-off and when the tube had been
placed inside the stomach. Feeding into the lung or oesophagus took place when a low pH (≤cut-off) was combined
with tube misplacements. If the pH exceeded a certain
cut-off, then chest X-rays were used to establish tube sites.
For those patients with tubes placed inside the stomach,
the X-ray was deemed unnecessary because gastric placement could have been determined solely by pH. We
distinguish between feeding by pH and feeding by chest
X-rays since the latter carries radiation risks and could
cause feeding delays for up to 47 hours.iii The remaining
patients who received chest X-rays would reveal misplaced
tubes—these were correctly identified and excluded, thus
no feeding outside the stomach.
The safest outcomes (ie, feeding into the stomach by
pH, no feeding outside the stomach) were assigned a
score of 100 and the least safe outcomes (ie, feeding into
the lung) was assigned a score of 0. For the remaining
outcomes, we applied the analytic hierarchy process,14
converting qualitative judgements into quantitative
scores. Two clinicians who were experts in gastroenterological diseases were invited to a face-to-face meeting with
one of the authors (MN). During the meeting, they were
briefed about the project and asked to first rank all the
outcomes according to safety. They were then asked to
make pairwise comparisons and articulate the strength of
their preferences. For instance, feeding into the oesophagus was considered safer than feeding into the lung and
the preference was very strong.
Consensus was reached through discussions, producing
preference judgements ranging from no difference,
weak, moderate, strong to extreme. We entered these
into the MACBETH15 component of the decision analysis
software, HiView. The software first checked whether the
judgements were consistent with the safety rankings and
once satisfied, converted the judgements into numeric
ratings (table 1, last column). Further consistency checks
were performed on the scores. For instance, oesophageal feeding received a safety score of 45, which means
that its safety was considered nearly halfway in between
the safest outcome (stomach feeding) and the least safe
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
Table 1
pH test
Probability and safety of decision outcomes of the
Outcome
Probability
Score
Feeding into the
stomach by pH
Feeding into the lung
by pH (feeding error)
Prior probability of
100
stomach×Sensitivity of pH
Prior probability of lung×(1—
0
Specificity in lung)
Feeding into the
oesophagus by pH
(feeding error)
Prior probability of
oesophageal×(1—Specificity
in oesophagus)
45
Delayed feeding into
the stomach by X-rays
(unnecessary X-rays)
No feeding outside the
stomach by pH or by
X-rays
Prior probability of
stomach×(1—Sensitivity of
pH)
Prior probability of lung/
oesophagus×Specificity in
lung/oesophagus
85
100
outcome (lung feeding). This should mirror the pairwise
comparisons, where the preference for stomach feeding
over oesophageal feeding (100 vs 45) was slightly stronger
than the preference for oesophageal feeding over lung
feeding (45 vs 0).
Outcome probabilities were driven by two independent factors—the initial insertion (prior distribution
of tube sites) and the accuracy of the pH test in differentiating various tube sites (ie, test sensitivity and specificity, table 1, middle column). Given the wide range
of variations in reported tube misplacements and tube
migrations (1.3%–50%), we assumed an average risk of
insertion errors, whereby 70% of the tubes were inside
the stomach with an equal number (15%) of misplacements in the lung and oesophagus (see online supplementary appendix 1 for reasoning). The sensitivities and
specificities under individual pH cut-offs were extracted
from our previous research. They were based on a clinical
database with 1035 unique patient records from multiple
clinical trials by a single clinician. This database included
754 stomach placements and 281 lung placements,16 17
with pH measured by both pH metre (Beckman pH1 10
portable pH metres) and pH paper throughout (1–11
Vivid pH paper). Since pH metre reading and paper
reading do not always agree, we used pH metre reading
to derive the accuracy data. Lack of evidence for oesophageal placements was remedied by reviewing studies on
healthy cohorts under observations for reflux.18–20 Distribution of oesophageal pH was estimated based on the
proportion of time when pH decreased below the various
cut-offs. Table 2 summarises the accuracy of pH tests.
Aggregating outcome scores by their respective probabilities resulted in a set of weighted scores. These reflected
the relative safety of the recommended checking procedure under different pH cut-offs. In addition to the pH
test, we analysed a scenario where patients are fed without
safety checks (feed all) and where all patients are sent for
chest X-rays before feeding (routine X-rays).
3
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Table 2 Accuracy of pH test under cut-offs 1–9
pH cut-offs
Sensitivity
(stomach)
Specificity
(lung)
Specificity
(oesophagus)
1
2
0.015
0.257
1
1
1
1
3
0.39
1
1
4
0.544
1
0.985
5
0.68
1
0.948
5.5
0.743
1
6
0.81
7
8
9
No. of cases
pH test correctly carried out but invalid
(pH<5.5 but tube not in stomach)
pH test wrongly interpreted (thought OK if
pH=6)
10
1
5
0.81
Bubble or Whoosh test used as only checking
procedure
2
0.996
0.792
CXR incorrectly interpreted
0.914
0.91
0.492
0.337
0.004
0.225
0.068
Correct test indicated tube in stomach but
tube moved prior to starting feed
4
0.991
1
No action taken to assess tube placement
12
CXR done but not checked prior to feeding
2
ReSulTS
Analysis of feeding incidents reported to nRlS
A total number of 2368 adverse event reports were identified. After excluding cases that were irrelevant or with
incomplete information, we reviewed 104 cases with
documented feeding tube misplacement. These included
There is no reported data on the frequency of chest X-ray misinterpretations for verifying NG tube insertions. Reported error rates of diagnostic X-rays for lung cancers ranged between 5.3% and 24%. Since
chest X-ray is a second-line test, the greater the likelihood of its misinterpretations, the more a higher rather than lower pH cut-off would be
preferred.
4
Mode of failure to identify tube misplacement
Type of failure
Aspiration used as checking procedure;
unclear whether pH tested
Sensitivity analyses
To capture the spectrum of insertion errors, we analysed
two additional scenarios with low (10%) and high (50%)
probability of tube misplacements (see online supplementary appendix 1 for reasoning). Lung and oesophageal
intubations were equally likely at 5% and 25%, respectively. Tornado diagrams were used to identify variables
of importance. All outcome and probabilistic inputs
were varied ±15% within range (0–1 for probabilities and
0–100 for outcomes). Three-way sensitivity analyses were
carried out to examine the direction of impact.
We considered the impact of successful aspirations,
whereby aspirations were successful 90% of the time.21
For the remaining 10%, chest X-rays are used instead. We
considered the impact of chest X-ray misinterpretations
by assuming that chest X-ray of tubes located outside the
stomach was interpreted as inside 10% of the time, which
resulted in feeding into the wrong places (equally likely
in lung and oesophagus)[v]. We then analysed the joint
impact of unsuccessful aspirations combined with radiography misinterpretations.
We carried out the analyses in Microsoft Excel and
TreeAge Pro (2015). Since chest X-rays were used as the
reference standard across pH accuracy studies, chest
X-rays were assumed to be 100% accurate in the main
body of analysis.
v
Table 3
25
Other (misinterpretation of CXR report) (CT
3
scan misreported)(direct vision and no further
checks)
Total
64
6 counts of death, 15 counts of severe harm and 23 counts
of moderate harm. The remaining 60 cases recorded no
harm (43 cases) or low harm (17 cases). Further analysis
was carried out on 75 out of 104 narratives containing
sufficient details. In 11 reports, the wrong tube location
was discovered prior to feed or medication (either by pH
or by chest X-rays). Of the remaining 64 cases, we analysed
reasons for misfeeding. The most frequently cited reason
was misinterpretation of chest X-rays (25 cases). The pH
test (with 5.5 as the cut-off) itself was responsible for 10
feeding incidents. There were also 23 cases where safety
guidelines were not followed, including 12 cases where
feeding was carried out without safety checks (table 3,
online supplementary appendix 2 for further details).
Safety of the ph test under various cut-offs
The higher the cut-off, the more sensitive and the
less specific the pH test becomes (table 2). However,
it is impossible to be free from X-ray-related feeding
delays and at the same time to be free from feeding
incidents due to the lack of accuracy of the pH test.
This is captured in figure 2, which shows the trade-off
between feeding incidents (more numerous under
higher cut-offs) and feeding delays (more numerous
under lower cut-offs). As the cut-off reduced, the
increase in the number of unnecessary X-rays (feeding
delays, x axis) was faster than the reduction in feeding
incidents (y axis). Consider cut-off 5 vs 6 for instance.
The magnitude of difference was four times, that is 9%
(=22%–13%) increase in unnecessary X-rays versus 2.1%
(=2.9%–0.8%) decrease in feeding incidents (primarily
in the oesophagus).
Nevertheless, it would be misleading to select pH
cut-offs based on the number of feeding delays or
feeding incidents alone because different outcomes
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
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misfeeding in 15% oesophageal placements (part
score 6.75) and in 15% lung placements (part score 0).
Routine use of chest X-rays had a weighed score of 89.5
from correctly identifying all 30% of misplaced tubes
(part score 30) and from feeding correctly in 70% of
the patients though with a delay (part score 59.5, mean
delay 17 hours, range 1.5–47 hours,iii see online supplementary appendix 3 for further details).
Figure 2 Trade-off between the number of unnecessary
X-rays and feeding incidents.
have different impact on patient safety. Instead, we
used the aggregated safety scores to assess the relative safety under different cut-offs. These are shown in
figure 3, along with part-score contributions made from
individual outcomes. At lower cut-offs, the scores were
primarily made up of delayed feeding and no feeding
outside the stomach, whereas at higher cut-offs stomach
feeding made increasingly significant contribution to
the overall safety. No points were attributed to lung
feeding with a safety score of 0.
Cut-off 5 and cut-off 6 had the highest safety score
(96.2), and, therefore, the ‘safest’ overall. This is in a
context of an ideal (and hypothetical) test which has a
score of 100, by identifying every tube in the stomach for
feeding while excluding every tube outside the stomach.
By contrast, to feed all patients without discrimination
is the least safe strategy with a weighted score of 76.75,
from feeding correctly (though randomly) in 70% of
patients with stomach placements (part score 70) but
Sensitivity analysis
The largest impact on the overall safety was attributable to safety of delayed feeding (scores 50–95) and to
the pH specificity in the oesophagus (range 0.6–0.99).
Decreasing the score assigned to delayed feeding by five
points (from 85 to 80) would make cut-off 5, the safest
option. A 10% increase at 5.5 (from 0.81 to 0.89) while
keeping the specificities at 5 constant (0.948) would
result in cut-off 5.5 becoming the safest overall. Varying
the initial tube misplacements also had a large impact,
influencing safety across all cut-offs. However, cut-off 5
remained the ‘safest’ under 50% tube misplacements.
Similarly, unsuccessful aspirations and/or chest X-ray
misinterpretations reduced the safety across all cut-offs
and more so for lower cut-offs than for higher cut-offs,
since chest X-rays were used more often at lower cut-offs.
Despite this, pH test under cut-off 5 remained the safest
within range 1–5.5 (table 4).
DISCuSSIOn
Summary of main findings
The recommended safety procedure prior to feeding by
NG tube is composed of two tests, the pH test and chest
X-rays when the pH test fails (>5.5). Our analysis showed
that with a score of 96.2 out of 100, the checking procedure was the safest under cut-off 5 given 30% or more
of tube misplacements. Respiratory feeding is excluded;
misfeeding in the oesophagus was kept to a minimum to
balance the need to reduce feeding delays from unnecessary chest X-rays. Routine chest X-rays were less safe than
the pH test (score 89.5) and to feed all was the most risky
(score 76.76).
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.
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
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Table 4 Sensitivity analysis of the safety of various pH cut-offs
Cutoffs
Value of
Value of
Oesophagus
Value
delayed
delayed
specificity inc.
(original) feeding=50 feeding=95 by 10% at 5.5
Initial
Initial
misplace- misplace- Unsuccessful CXR
ment=10% ment=50% Aspirations misinterpretation
1
2
89.7
92.2
65.5
74.0
96.6
97.4
89.7
92.2
86.7
90.0
92.6
94.4
86.7
89.2
86.6
88.9
3
93.6
78.7
97.9
93.6
91.8
95.4
90.6
90.2
4
95.1
83.9
98.3
95.1
93.8
96.4
92.1
91.5
5
96.2
88.4
98.5
96.2
95.5
96.9
93.3
92.6
5.5
95.7
89.4
97.5
96.4
96.0
95.5
93.0
92.4
6
96.2
91.6
97.6
96.2
96.8
95.6
93.5
92.8
7
93.6
91.4
94.2
93.6
97.0
90.1
91.5
91.0
8
9
83.6
77.4
83.3
77.4
83.6
77.4
83.6
77.4
94.4
92.5
72.7
62.3
82.7
77.3
83.1
78.2
Strengths and limitations
Using a decision analytic approach, we analysed the safety
of the checking procedure under pH cut-offs 1–9 based
on combined evidence from expert judgements, literature and clinical studies. We considered both the impact
and the probabilities of various outcomes. Feeding delays
caused by chest X-rays were formally incorporated, by a
safety score lower than the ideal 100. The entire range
of pH cut-offs was analysed, in addition to the commonly
used ones. The safety of routine chest X-rays and feeding
all patients without checks was similarly analysed.
The key evidence base underlying this analysis comes
from Metheny et al over a 12-year period (1989–2001).
Although slightly dated, this research, we believe, remains
the most impressive body of evidence on aspirate pH
measurement and prediction of feeding tube position by
using a standard well-designed study protocol from six
acute care hospitals.
The largest uncertainty remains in the oesophageal pH,
especially in the critical range between cut-offs 4 and 5.5
due to the lack of direct evidence. We did not consider
costs in this analysis. However, the same framework can
be applied when new evidence becomes available and
extended to incorporate additional factors of importance, for example, costs. A further limitation of our
study is that our evidence on gastric and respiratory pH
came from pH metre measurement whereas in practice
pH papers are widely used. This will not influence our
conclusion because pH paper is known to be less sensitive when compared with pH metre. In addition, we have
focused only on the subgroup of patients with successful
aspirations since we are primarily interested in the relative safety of pH cut-offs. Our sensitivity analysis explored
impact from unsuccessful aspirations and chest X-ray
misinterpretations. Although safety across all pH cut-offs
has been reduced, cut-off 5 remains the safest test to use.
Comparison with existing literature
As a universal first-line test for ensuring feeding safety,
numerous studies investigated the pH test for its accuracy
6
in identifying stomach and lung placements.16 17 However,
all the studies focused on the accuracy of the pH test per
se. By contrast, the checking procedure, in fact, contains
two tests: the pH test followed by chest X-rays when necessary. Thus, the safety of the pH test must be evaluated
in the context of its use, by considering its downstream
implications for clinical decision-making. We found that
the key issue was to achieve a balance between reducing
feeding incidents and reducing unnecessary chest X-rays.
The decision analytic approach provides the normative
framework for dealing with conflicting objectives. One
study closer to our remit22 investigated cost utility of the
clinical algorithm (ie, checking procedures) for NG tube
placement confirmation in adult patients. Our study
differs from this study in that in our study accuracy of the
pH test is not a given but constitutes the key source of
uncertainties for achieving safe NG feeding.
Implications for practice
Although the current recommended pH cut-off is 5.5, the
British Society of Gastroenterology guidance for enteral
feeding suggests that tube aspirate pH measurement
needs to be <5.0 prior to every use, but advises caution
when the patient is on acid suppression.23 Routine use of
X-rays was not advised. Our study showed that reducing
the pH cut-off from 5.5 to 5 can reduce the number of
feeding incidents. Because majority of the patients have
stomach placements, and because gastric pH has a mean
value around 4,8 a lower threshold means that more
patients will be sent for chest X-rays. Chest X-rays, when
misinterpreted, can lead to feeding incidents.7 There is a
clear need to develop cost-effective bed-side tests which
not only have high accuracy but also have the ability to
withstand human errors in their applications.
Chest X-rays misinterpretations
Although chest X-ray misinterpretations constituted a
major source of feeding errors (table 3), our main analysis
assumed that chest X-rays were 100% accurate based on a
number of considerations. First, we limited our evidence
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
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base to a cohort of clinical studies with clear demonstration of administering and interpreting a reference standard (see above). This gave us confidence in the accuracy
of chest X-rays in our evidence base. Second, there is little
data on the actual distributions of chest X-ray misinterpretations in relation to NG tube feedings, obscuring the
direction in which the analysis would be influenced by
such an assumption. A stomach tube might be misinterpreted as located outside the stomach, resulting in either
overestimation of the specificity of the pH test (when
pH>5.5) or underestimation of test sensitivity (when
pH<5.5). Similarly, a non-stomach tube might be misinterpreted as located inside the stomach, with opposite
implications for test sensitivity and specificity.
It is also important to note that in practice, reducing
pH cut-offs from the existing 5.5 to 5 will not increase
misfeeding attributable to chest X-ray misinterpretations.
This is because the change will affect those patients with
a pH between 5 and 5.5. All these patients will receive
feeding under the existing cut-off, whereas under the
new, lower cut-off, only a proportion of them, who have
demonstrated stomach intubation from chest X-rays, will
be fed. An important lesson here is that the quality of a
formal analysis is inevitably constrained by the availability
of the evidence, and the quality of it.
COnCluSIOnS
The pH test with an upper cut-off at 5 was the safest test
for the verification of NG tube locations. The choice of
pH cut-off depended on the prevalence of tube misplacements, the impact of feeding delays and the specificity
of the pH test for oesophageal placements. Routine data
collection at the local level should be implemented to
optimise safety recommendations.
Contributors GBH, PMMB, LDP and MZN designed the study; MZN, OHP and SO
conducted research; OHP and SO analysed the NRLS database. MN, OHP and SO
interpreted the data. MZN carried out the decision analysis. MZN, JRH and PMMB
drafted the manuscript. Final manuscript is approved by all authors. GBH had final
responsibility for the final content.
Funding The research was supported by the NIHR Diagnostic Evidence
Co-operative, London. The views expressed are those of the author(s) and not
necessarily those of the NHS, the NIHR or the Department of Health. MZN is also
partially supported by a grant from the Innovate UK (Biomedical Catalyst Fund, ref:
102134) award to GBH and company Ingenza, for developing a new pH paper test
for nasogastric tube placements.
Competing interests None declared.
ethics approval London—Chelsea REC(Reference: 16/LO/0998), which supported
a clinical study as well as interview studies related to the use of pH tests in the
clinical setting.
Provenance and peer review Not commissioned; externally peer reviewed.
Data sharing statement No additional data availale
Open Access This is an Open Access article distributed in accordance with the
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permits others to distribute, remix, adapt, build upon this work non-commercially,
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licenses/by-nc/4.0/
Ni MZ, et al. BMJ Open 2017;7:e018128. doi:10.1136/bmjopen-2017-018128
© Article author(s) (or their employer(s) unless otherwise stated in the text of the
article) 2017. All rights reserved. No commercial use is permitted unless otherwise
expressly granted.
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7
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Selecting pH cut-offs for the safe verification
of nasogastric feeding tube placement: a
decision analytical modelling approach
Melody Zhifang Ni, Jeremy R Huddy, Oliver H Priest, Sisse Olsen,
Lawrence D Phillips, Patrick M M Bossuyt and George B Hanna
BMJ Open 2017 7:
doi: 10.1136/bmjopen-2017-018128
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http://bmjopen.bmj.com/content/7/11/e018128
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