A team of researchers from University of California - Los Angeles and Harvard University demonstrated the technique, which uses a deformability cytometer to analyse individual cells.
It could reduce the need for more cumbersome diagnostic procedures and the associated costs, while improving accuracy over current methods.
The study analysed pleural fluid samples from more than 100 patients.
Pleural fluid, a natural lubricant of the lungs as they expand and contract during breathing, is normally present in spaces surrounding the lungs.
Medical conditions such as pneumonia, congestive heart failure and cancer can cause an abnormally large buildup of the fluid, which is called a pleural effusion.
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Preparing cells for this analysis can involve complicated and time-consuming dyeing or molecular labelling, and the tests often do not definitively determine the presence of tumour cells.
The new method, developed previously by the UCLA researchers, requires little sample preparation, relying instead on the imaging of cells as they flow through in microscale fluid conduits.
To understand the method imagine squeezing two balloons, one filled with water and one filled with honey. The balloons would feel different and would deform differently in your grip, researchers said.
The amount of a cell's compression can provide insights about the cell's makeup or structure, such as the elasticity of its membrane or the resistance to flow of the DNA or proteins inside it.
Cancer cells have a different architecture and are softer than healthy cells and, as a result, "deform" differently.
Using deformability cytometry, researchers can analyse more than 1,000 cells per second as they are suspended in a flowing fluid, providing significantly more detail on the variations within each patient's sample than could be detected using previous physical analysis techniques.