pH-metry of amniotic fluid: diagnostic value and interpretation

Measuring the pH of amniotic fluid and fetal scalp blood is used in obstetric practice to confirm two objectives: confirming rupture of membranes and assessing the fetal acid-base balance when intrauterine hypoxia is suspected. These tests complement clinical examination and continuous cardiotocography and help decide whether to deliver immediately or continue labor. [1]

Historically, pH measurements emerged following the spread of cardiotocography as an attempt to obtain more objective information about the fetus's condition in the presence of abnormal heart rate patterns. The practical goal is to distinguish false alarms from actual hypoxia, thereby reducing the number of unnecessary cesarean sections while simultaneously preventing fetal distress. [2]

Today, clinical guidelines consider pH-metry as an auxiliary, secondary method. Decisions are made based on a comprehensive assessment: the clinical situation, the progress of labor, echocardiography and cardiotocography data, as well as the availability of the technique and the qualifications of the personnel. [3]

None of the listed tests are perfect. It is important to know the limitations of each method, both technical and biological factors, and to use the results with caution, in combination with other clinical data. [4]

Physiology and logic of pH control during labor

Amniotic fluid has a physiologically more neutral pH than the upper vagina, and therefore an increase in vaginal pH serves as a marker for amniotic fluid leakage. However, other fluids, such as urine, can alter the results, so ancillary tests require careful interpretation. [5]

Fetal blood reflects its acid-base balance and oxygenation. With increasing hypoxia, metabolic acidosis develops, manifested by a decrease in blood pH. Therefore, direct sampling of capillary blood from the fetal scalp during birth allows for an assessment of the fetus's actual metabolic status at the time of testing. [6]

A decrease in pH to certain thresholds correlates with an increased risk of adverse perinatal outcomes. In obstetric practice, thresholds are generally accepted and used for decision-making, but modern research continues to refine optimal boundaries and predictive value. [7]

Understanding the pathophysiology is also important for interpreting the results: acute hypoxia leads to rapid lactate accumulation and a decrease in pH, while chronic hypoxia produces different patterns. The diagnostic test should be perceived as a "snapshot" of the condition, not as the sole basis for decisions. [8]

Methods for measuring pH of amniotic fluid - possibilities and limitations

In practice, nitrazine paper or ready-made pH pads are used to detect amniotic fluid leakage. The simplicity of this method makes it common in maternity wards. However, its sensitivity and specificity are limited: contamination with urine, menstrual or vaginal secretions, and bacteria can produce false positive results. [9]

Alternative methods for diagnosing membrane leakage include the petrifix test and microscopic ferning, as well as commercial enzymatic reaction-based panels. Recent reviews emphasize that the combined use of methods improves accuracy compared to a single test. [10]

The pH test does not directly indicate fetal status. Even if membrane retardation is confirmed, cardiotocography and clinical findings should be used to guide management decisions. Nevertheless, the rapid pH test remains a practical tool for initial assessment. [11]

When interpreting the results, it is important to consider the time since the leak and possible contamination of the sample. Laboratory analysis of amniotic fluid provides more accurate data, but it is time-consuming and not always available during labor. [12]

Table 1. Threshold values of vaginal pH in cases of suspected vaginal leakage

Test result	Interpretation	Comments
pH ≤ 6.5	Most likely there is no leakage	Possible acidic vaginal environment. [13]
pH 6.5-7.0	Ambiguous result	Further verification is recommended.[14]
pH ≥ 7.0	Most likely amniotic fluid leakage	Consider possible contamination of the sample. [15]

Technique for collecting blood from the fetal scalp and analysis

A fetal scalp biopsy is performed transvaginally when the cervix is dilated and a sufficient portion of the head is present. After disinfection and removal of mucus, capillary blood is collected using a special needle. A clean, homogeneous sample free of amniotic fluid is required for reliable results. [16]

The collected capillary blood is analyzed promptly for pH or lactate concentration. Both parameters are informative: pH reflects overall acid-base status, while lactate more specifically indicates metabolic acidosis. Some centers are moving toward measuring lactate as a more practical and reproducible parameter. [17]

Classic pH thresholds frequently used in clinical guidelines are: normal pH > 7.25, borderline pH 7.20–7.25, and abnormal pH < 7.20. These thresholds are used to guide decisions about delivery in the presence of abnormal cardiotocograms. Current research continues to evaluate the sensitivity and specificity of these thresholds. [18]

The procedure requires skill. Technical errors, air ingestion, hemolysis, contamination with amniotic fluid or vaginal secretions, and delays in analysis distort the results and can lead to erroneous clinical decisions. Therefore, preparation and quality control standards are critical. [19]

Table 2. Classical thresholds for fetal scalp blood pH and clinical actions

pH	Interpretation	Recommended tactics
> 7.25	Fine	Continue monitoring and adjust according to CTG. [20]
7.20-7.25	Borderline	Assess the entire clinical picture; if CTG deteriorates, consider timely delivery. [21]
< 7.20	Pathological	Consider immediate delivery depending on the progress of labor and the mother's condition. [22]

Alternatives and additions

In recent years, interest has grown in measuring fetal scalp lactate as an alternative to pH. Lactate more often correlates with metabolic acidemia and, in some studies, demonstrates better reproducibility. Some regulatory documents allow the use of lactate as a substitute in settings where the method is available. [23]

ST analysis of the fetal electrocardiogram signal and other computer-based methods of cardiac activity analysis provide continuous data and may reduce the need for invasive testing. However, large randomized trials have shown mixed results regarding the impact on cesarean section rates and perinatal outcomes. Therefore, these technologies are considered a complement to, rather than a replacement for, targeted blood pH testing. [24]

Non-invasive methods continue to evolve: intrafetal pulse oximetry and innovative sensors for real-time pH measurement are in clinical trials. While they have not yet become standard, they could potentially reduce the number of invasive procedures. [25]

The choice of method should take into account availability, personnel qualifications, speed of obtaining results, and the level of evidence. A combined approach—CTG plus targeted pH assessment if necessary—remains the most justified today. [26]

Table 3. Comparison of the main methods for assessing the acid-base balance of the fetus during labor

Method	Invasiveness	Speed	Reliability	Comments
pH of fetal scalp blood	Invasive	Fast	Good if fenced correctly	Requires skill. [27]
Lactate in the blood of the fetal scalp	Invasive	Fast	High reproducibility	More and more popular. [28]
ST analysis	Non-invasive	Continuously	Varies according to studies	Supplement to CTG. [29]
Fetal pulse oximetry	Non-invasive	Continuously	Limited clinical benefit	Under investigation. [30]

Pre-analytical and analytical errors - how to avoid being misled

Preanalytical errors include contamination with amniotic fluid, maternal blood, hemolysis, and air ingestion. Any of these errors can artificially increase or decrease the pH, leading to incorrect interpretation. [31]

The analytical field requires calibrated instruments and quality control standards. Rapid gas analyzers used during labor should be regularly calibrated, and personnel should be trained in handling small volumes of blood. [32]

The interpretation of the result must always take into account the context: one normal or one abnormal test does not replace dynamic observation. The decision to deliver the baby is based on a combination of clinical, dynamic, and laboratory data. [33]

Logistics – the time from collection to analysis should be minimal. Delays, transportation, and improper storage of samples reduce reliability. Therefore, when planning the implementation of the method, it is important to consider the workflow and team training. [34]

Table 4. List of pre-analytical factors that distort the pH result

Factor	Influence
Amniotic fluid contamination	May change the pH of the sample.[35]
Ingestion of maternal blood	Greatly distorts the result. [36]
Hemolysis	Impairs measurement accuracy. [37]
Air bubbles in a test tube	May lower pH and cause error.[38]

A Practical Decision-Making Algorithm - How to Use pH Data

The algorithm begins with abnormal cardiotocography. If hypoxia is strongly suspected and the data are inconsistent, blood is collected from the fetal scalp. If pH is < 7.20 or lactate is above established thresholds, immediate delivery is considered; if values are borderline, the dynamics and clinical picture are taken into account. [39]

It is important that blood sampling does not delay a timely decision. If the procedure is technically impossible or protracted, action is taken based on the clinical picture and CTG. The test should not interfere with operative delivery if there are clear signs of fetal distress. [40]

If the pH is normal and the CTG dynamics are stable, labor continues under observation. If borderline values recur, frequent monitoring and discussion with the labor team are required. Documentation of each step is mandatory. [41]

The algorithm needs to be adapted to local protocols and resources. In departments where the method is rarely used, it is useful to have a checklist and regularly train the team. [42]

Table 5. Simplified clinical algorithm for abnormal cardiotocography

Step	Action
1	Assess the mother and fetus. Quick ultrasound and CTG review. [43]
2	If hypoxia is suspected and blood sampling is possible, perform FSBS and measure pH or lactate. [44]
3	pH ≥ 7.25 - continue observation; pH 7.20-7.25 - evaluate the dynamics; pH < 7.20 - consider delivery. [45]
4	If collection is not possible, make a decision based on CTG and the clinical picture. [46]

Limitations of the evidence base and prospects

PH measurement methods have been used in clinical practice for decades, but their predictive value depends on the quality of studies and standardization of methods. Modern systematic reviews indicate the usefulness of pH and lactate in certain situations, but there are no universal recommendations that are mandatory for all centers. [47]

New technologies and biosensors are emerging that promise continuous monitoring of acid-base parameters and a reduction in the proportion of invasive tests. Clinical trials of these devices are ongoing and may change practice in the coming years. [48]

The key guiding principle remains the same: any laboratory test is part of a clinical context. There should be no automatic reliance on numbers without interpretation by an obstetrician and discussion with the team. [49]

It is important for guiding practice to support local protocols, train staff, and participate in clinical research to improve the quality of evidence and safety of childbirth. [50]

Table 6. Practical recommendations for implementing the method in the maternity ward

Step	Recommendation
1	Develop a local protocol and checklist for FSBS. [51]
2	Train the team and conduct regular training. [52]
3	Provide access to rapid pH and lactate analysis. [53]
4	Document results and decisions, audit outcomes. [54]

Conclusion - What is important for a practicing physician to remember

Amniotic fluid pH monitoring remains a simple means for the initial diagnosis of membrane leakage, but its results require careful interpretation due to the risk of contamination. [55]

Fetal scalp blood sampling provides valuable information about fetal metabolic status. pH and lactate thresholds aid clinical decision-making, but technique and pre-analytical conditions are critical for reliability. [56]

Alternative and complementary methods, including lactate, ST analysis, and new sensors, are developing. Currently, the optimal strategy is the integration of cardiotocography data with targeted pH assessment when indicated. [57]

When implementing the method into practice, it is necessary to have local protocols, trained personnel, and a quality control system. This will reduce the number of false interpretations and improve perinatal outcomes. [58]