Determination of vaginal pH by pH indicator strip and by pH

Heinze et al, Vaginal pH measured by indicator strip and micro electrode
477
Short communication
J. Perinat. Med.
17 (1989) 477
Determination of vaginal pH by pH indicator strip and by pH
micro electrode
Thomas Heinze, Susanne Riedewald, and Erich Saling
Institute of Perinatal Medicine, The Free University of Berlin and Department
of Obstetrics, Women's Hospital Berlin-Neukölln, Berlin, West Germany
1
Introduction
Prematurity remains the predominant cause of
perinatal mortality and morbidity. One cause of
prematurity is the bacterial infection of the vagina. A large number of microbial organisms are
present in the vaginal fluid. Some of them such
as group B-streptococcus, chlamydia trachomatis, mycoplasma hominis etc. have been implicated as causative agents.
The epithelial cells of the normal adult vagina
are rich in glycogen, which influence the growth
of microorganisms. The high acidity of the vaginal fluid is influenced mainly by the lactic acid
produced by the metabolism of lactobacilli
(Döderlein bacilli). Bacterial infection is associated with a less acid pH [4]. Furthermore clinical
observations show a trend towards higher vaginal pH values in association with preterm labour
and with preterm rupture of the membranes [1,
2,5].
In this study we used two different methods of
pH measurements in freshly taken vaginal fluid
and proved their suitability as screening methods
for vaginal infection as a possible risk factor for
preterm delivery.
2
Materials and methods
The study included 84 pregnant women suffering
from preterm labour, preterm rupture of the
membranes, vaginal bleeding or other obstetric
diseases without complaints or signs of bacterial
vaginitis. Vaginal pH measurements were performed between the 8th and 39th gestational
week. The measurement with pH electrodes is
reliable and precise, but the direct measurement
1989 by Walter de Gruyter & Co. Berlin · New York
in the vagina creates the problem of sterilising
the electrodes and the demand for some more
electrodes if the method is used as a screening
method. This is why we choose the pH measurement in vitro i. e. we measured the pH in freshly
taken vaginal fluid.
During a routine vaginal examination a sterile
speculum was used to expose the posterior vaginal fornix. After examination the vaginal fluid
is concentrated in the lower speculum. In cases
with only a very small amount of vaginal fluid
we tried to obtain more by cautiously collecting
some fluid with the lower speculum in the posterior fornix. Special pH indicator strips nonbleeding range 4.0-7.0 (No. 9542 Merck Co.,
Darmstadt, FRG) were compared with a colour
scale which allows to measure the pH in steps of
0.2 — 0.5 pH units. However, it is possible to
estimate shades of colours between the steps
given on the scale.
To avoid a subjective influence in the pH determinations by assessing the colour of the indicator
strip, we first carried out the measurement with
pH indicator strip, registered the values and secondly performed the measurement with the micro electrode. We used a micro pH combination
electrode with a Ag/AgCl reference electrode
(type 406 M 3, Ingold Co., Steinbach, FRG) in
connection with a pH meter (type Portamess 902,
Knick Co., Berlin, FRG). The accuracy of the
pH measurement by indicator strip is about 0.1
to 0.2 and by electrode 0.01 pH units. The glass
electrode was calibrated at pH 7.0 and pH 4.O.
In cases where there is only a small amount of
vaginal fluid available, pH measurement is possible if the tip of the electrode up to the exit of
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Heinze et al, Vaginal pH measured by indicator strip and micro electrode
the reference electrode is completely covered by
a film of the vaginal fluid. A minimum of 50 —
100 μΐ (i.e. 1—2 drops) is required for the pH
measurement with indicator strip on its own,
when performing both methods, we need 200 —
300 μΐ vaginal fluid. In a very small number of
cases we failed in getting enough vaginal smear
for both determinations or the measurement was
prohibited by vaginal bleeding.
3
7-,
6-
5-
y = 0.994 χ + 0.036
r = 0.9633
£
ι
Results and discussion
The figure shows the results of 84 measurements
of pH values by both methods. We received a
highly significant correlation between both methods (p = 0.0000) in the range from pH = 4.0 to
7.0. The pH measurement by the indicator strip
we used is limited in the range from pH = 4.0
to 7.0. Values under pH = 4.0 show a normal
vaginal flora and are not of further clinical interest.
The advantages of the vaginal pH determination
by indicator strip are: it is easy to handle and
low in cost, requires a very small amount of
vaginal fluid, excludes the risk of infection and
needs no calibration. The disadvantage is the
relatively low accuracy of the pH determination
(0.1—0.2 pH units), caused by subjective interpretation of the colour of the indicator strip.
The advantages of the micro glass electrode are:
the good accuracy (0.01 pH units), no problems
with subjective interpretation, the small amount
of vaginal fluid (ca. 200 μΐ) needed and by performing the measurement in vitro no risk of
infection either.
pH indicator strip
Figure 1. pH values determined by pH micro glass
electrode compared to those by indicator strip. Linear
regression and standard deviation.
We recommend the pH determination of the
vaginal fluid by indicator strip in combination
with the micro glass electrode as a screening
method for vaginal infection. Risks of preterm
labour and preterm delivery increased when the
vaginal pH is higher than 4.3 [5] resp. 4.4 [2] or
4.5 [3]. Therefore up to a pH value =4.3 it is
sufficient to use indicator strips. In all cases with
a possible risk of infection (pH values > 4.3)
and in all cases where a precise determination of
the pH value is needed, the micro electrode
should be used.
In cases where measurements are necessary at
particular points of the vagina e. g. in case of
preterm rupture of membranes a direct measurement inside the vagina with a pH electrode according to [5] must be carried out.
Abstract
Vaginal pH measurement is a screening method of
vaginal infection in pregnancy. We tested two different
methods of pH determination. In freshly taken vaginal
fluid measurements were carried out with special pH
indicator strips and pH micro glass electrode. About
100 μΐ of vaginal fluid is enough when pH indicator
strips are used, and 200 μΐ when the micro electrode
is used. The accuracy of the pH measurement by indicator strip is about 0.1—0.2 pH and by electrode
0.01 pH. Both methods show a good correlation between pH 4.0 to 7.0 (p = 0.0000). Until a pH of up to
4.3 we recommend the pH determination by indicator
strips. In all cases with the possible risk of infection
(pH values > 4.3) the micro electrode should be used.
Keywords: pH indicator strip, pH measurement, pH micro electrode, vaginal fluid.
Zusammenfassung
Bestimmung des vaginalen pH mit pH-Indikatorst bchen und pH-Mikroelektrode
Erh hte pH-Werte in der Vagina stehen in Zusammenhang mit bakteriellen Infektionen, vorzeitiger Wehen-
t tigkeit und vorzeitigem Blasensprung. Um die Anwendbarkeit der pH-Messung als Screening-Methode
zur Erkennung einer m glichen Fr hgeburt zu berpr fen, testeten wir zwei verschiedene Methoden der
J. Perinat. Med. 17 (1989)
Heinze et al, Vaginal pH measured by indicator strip and micro electrode
pH-Messung. Es wurden vaginale pH-Messungen bei
84 schwangeren Frauen zwischen der 8. und 39.
Schwangerschaftswoche durchgeführt. Im Rahmen
einer routinemäßigen vaginalen Untersuchung wurde
der pH-Wert in frisch entnommener Vaginalflüssigkeit
zuerst mit einem Indikatorstäbchen und dann mit einer
Mikroglaselektrode gemessen. Ca. 100 Vaginalflüssigkeit werden zur pH-Messung mit dem Indikatorstäbchen und 200 für die pH-Messung mit der pHElektrode benötigt. Die Genauigkeit der pH-Messung
bei Verwendung des Indikatorstäbchens liegt bei ca.
0,1 -0,2 und bei der Elektrode bei 0,01 pH-Einheiten.
Im Bereich von pH = 4,0 — 7,0 wurde eine sehr gute
Übereinstimmung
beider
Methoden
gefunden
(p = 0.0000). Die Vorteile der vaginalen pH-Messung
mit Indikatorstäbchen sind darin zu sehen, daß sie
leicht anwendbar sind, keine Eichung benötigen und
nur geringe Kosten entstehen. Außerdem wird nur ein
479
sehr geringes Probevolumen benötigt und das Risiko
einer Infektion ist ausgeschlossen. Der Nachteil ist die
relativ große Ungenauigkeit der pH-Bestimmung.
Die Vorteile bei der Verwendung der Mikroglaselektrode sind: die große Genauigkeit, es entstehen keine
Probleme durch subjektive Interpretation bei der Messung, es wird nur wenig Vaginalflüssigkeit benötigt und
das Risiko einer Infektion ist ebenfalls ausgeschlossen.
Aber die Methode ist etwas zeitaufwendiger und mit
größerem apparativen Aufwand verbunden.
Wir empfehlen die kombinierte pH-Messung mti pHIndikatorstäbhcen und mit der Mikroglaselektrode.
Bis zu einem pH-Wert von 4,3 ist es ausreichend,
Indikatorstäbchen zu verwenden. In Fällen einer möglichen Infektion (pH-Werte > 4.3) und wenn eine genaue pH-Bestimmung notwendig ist, empfiehlt sich die
pH-Bestimmung mit der Mikroglaselektrode.
Schlüsselwörter: pH-Indikator-Stäbchen, pH-Messung, pH-Mikroelektrode, Vaginalflüssigkeit.
Resume
Mesure du pH vaginal par bandelettes a pH et par microelectrode a pH
Les valeurs les plus elevees du pH vaginal
s'accompagnent d'infection bacterienne, d'accouchement premature et de rupture prematuree des membranes. Afm de prouver le caractere approprie de la
mesure du pH comme methode de depistage du risque
possible d'accouchement premature, nous avons teste
deux methodes differentes de mesure du pH chez 84
femmes enceintes entre la Seme et la 39eme semaines,
on a realise les mesures du pH sur des prelevements
vaginaux frais ä l'aide de bandelettes speciales indicatrices du pH et a l'aide de micro-electrodes. Le pH
a ete mesure in-vitro d'abord avec une bandelette, puis
ä l'aide d'une micro-electrode de verre apres examen
vaginal normal.
II suffit d'environ 100 de fluide vaginal pour mesurer
le pH a l'aide d'une bandelette et de 200 ä l'aide de
Pelectrode ä pH. La precision de la mesure du pH est
de 0,1 a 0,2 unites pH avec la bandelette et de 0,01
unite pH avec Felectrode.
II existe une bonne correlation pour les 2 methodes
pour les pH allant de 4 ä 7 (p = 0,0000). Les avantages
de la mesure du pH vaginal par bandelette sont les
suivants: elles sont faciles ä utiliser et peu couteuses,
elles necessitent tres peu de fluide vaginal, elles eliminent le risque d'infection et elles ne necessitent pas
de calibrage. Leur desavantage est la precision relativement faible de la mesure du pH. Les avantages de
la micro-electrode de verre sont les suivants: bonne
precision, absence de probleme d'interpretation subjective, faibles quantites du fluide vaginal necessaire et
absence de risque d'infection. Mais elles imposent un
equipement plus important et un temps plus long.
Nous recommandons la mesure du pH ä l'aide de
bandelettes indicatrices en association avec une microelectrode. L'utilisation de bandelettes est süffisante jusqu'a un pH de 4,3. Dans tous les cas ou il existe un
risque d'infection (pH 4,3) et dans tous les cas ou il
est necessaire d'avoir une mesure precise du pH, il faut
utiliser la micro-electrode.
Mots-cles: Bandelette ä pH, fluide vaginal, mesure du pH, micro-electrode ä pH.
References
[1] GLEESON RP, AM ALDER, MJ T\JRNER, AJ RUTHERFORD, MG ELDER: Vaginal pH in pregnancy
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Obstet Gynaecol 96 (1989) 183
[2] MINKOFF H, A GRUNEBAUM, J FELDMAN, M
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Obstet 25 (1987) 17
[3] PAAVONEN J: Physiology and ecology of the vagina.
Scand J Infect Dis Suppl 40 (1983) 31
[4] PEETERS F, R SNAUWAERT, J SEGERS, J VAN CUTSEM,
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[5] RJEDEWALD S, I-M KREUTZMANN, T HEINZE, E
SALING: Vaginal and cervical pH in normal pregnancy and pregnancy complicated by preterm labour. J Perinat Med (1989) in press
Received August 24, 1989. Revised October 19,
1989. Accepted October 30, 1988.
Dr. med., Dr. rer. nat. Thomas Heinze
Institut für Perinatale Medizin der FU Berlin
Mariendorfer Weg 28
D-1000 Berlin 44
West-Germany