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Pleural Fluid pH Test

Published: July 31, 2026Updated: August 16, 2026

The pleural fluid pH test measures the acidity of fluid that has collected in the space between your lungs and chest wall. This result helps your doctor find out why the fluid built up and choose the most effective treatment, such as whether chest tube drainage is needed.

Quick summary

{"Pleural fluid pH test measures how acidic the fluid around your lungs is","Low pH (below 7.2) may indicate infection, inflammation, or cancer","The test helps doctors decide if you need chest tube drainage or other treatments","Fluid is collected using a needle between your ribs (thoracentesis)","Results help distinguish between different types of pleural effusions"}

Medically reviewed by WellAlly Editorial TeamLast reviewed August 16, 2026Editorial policy

Key facts at a glance

LOINC
50895-2
Short code
pH Plr Strip
Synonyms
pH Plr Strip
Category
chemistry
Sample
serum
Container
Red-top tube (SST)
Units
not specified
Ref low
7.6
Ref high
7.64
Critical range
7.2 not specified – —
Fasting
false
Population
adult
Sex
all
Age range
18–65 years
Method
test-strip
Specimen volume
5 mL
Stability
Room temperature: 4h, Refrigerated: 24h, Frozen: 7d
Hemolysis max
500
Lipemia max
1000
Icterus max
500
TAT
4–24 hours
Order
routine

What is this test?

The pleural fluid pH test measures the acidity (pH level) of fluid that has accumulated in the pleural space—the area between your lungs and your chest wall. Normally, there is only a small amount of fluid in this space. When too much fluid builds up, it is called a pleural effusion. This test helps doctors understand why the fluid accumulated.

How is the sample collected?

A doctor will collect the pleural fluid using a procedure called thoracentesis. You will sit upright and lean forward on a table. The doctor will clean an area on your back, numb it with local anesthetic, and then insert a thin needle between your ribs to withdraw the fluid. The sample is then tested using a special strip that measures pH.

What do the results mean?

Pleural fluid pH results help classify the type of effusion: pH below 7.2 typically indicates an exudative effusion, which is often caused by infection (like pneumonia or tuberculosis), cancer, or inflammation. pH of 7.2 or higher suggests a transudate, which is usually caused by systemic conditions such as heart failure, liver disease, or kidney disease. The pH value also helps doctors determine if chest tube drainage is necessary.

Reference range

7.60-7.64not specified

Specimen: serum

No special patient preparation; collect anaerobically by thoracentesis and analyze immediately to avoid CO2 loss.

Clinical interpretation

How abnormal results are interpreted and what conditions they suggest.

Pleural fluid pH testing is clinically important because it helps differentiate between transudates and exudates, guiding treatment decisions. A low pH (<7.2) in pleural fluid is often associated with complicated parapneumonic effusions or empyema (pus in the pleural space), which typically require chest tube drainage and possibly antibiotics. In cancer-related effusions, low pH may indicate a more aggressive tumor. This test is a key tool in managing pleural effusions and determining the appropriate course of treatment.

Short name

PFpH

Frequently asked questions

Is the pleural fluid pH test painful?
You may feel some discomfort or pressure when the needle is inserted, but the area is numbed with local anesthetic to minimize pain. Most patients tolerate the procedure well.
How long does it take to get results?
Results are typically available within a few hours to a day, as the pH measurement is usually performed quickly at the bedside or in the laboratory.
Do I need to fast before this test?
No fasting is required for this test. You can eat and drink normally before the procedure.
Are there any risks to having pleural fluid removed?
Risks are minor but may include bleeding, infection, or a collapsed lung (pneumothorax). Your doctor will monitor you closely during and after the procedure.
Why does the doctor need to know the pH of the fluid?
The pH level helps determine the cause of the fluid buildup and guides treatment. For example, acidic fluid may require more aggressive treatment like chest tube drainage, while alkaline fluid may be managed with medication alone.
  1. 1
    Pleural Effusion: Diagnosis and ManagementSource
  2. 2
    ThoracentesisSource
  3. 3
    Pleural Fluid AnalysisSource

Click any link to access the full source. PubMed Central (PMC) links provide free full-text access.

This information is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment.

Medical Disclaimer

This content is for general informational and educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always seek the guidance of a qualified healthcare provider with any questions you have about a medical condition, and never disregard professional medical advice or delay seeking it because of something you read here.

If you think you may have a medical emergency, call your local emergency number or go to the nearest emergency department immediately.

Medically reviewed by WellAlly Editorial Team Last reviewed August 16, 2026. · Read our editorial policy