The Light Criteria: The Beginning and Why they are Useful 40 Years Later

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Key points

  • The Light criteria serve as a good starting point in the separation of transudates from exudates.

  • The Light criteria misclassify about 25% of transudates as exudates, and most of these patients are on diuretics.

  • If a patient is thought likely to have a disease that produces a transudative pleural effusion but the Light criteria suggest an exudate by only a small margin, the serum–pleural fluid protein gradient should be examined.

  • If this is greater than 3.1 gm/dL, the patient in all probability

The development of the Light criteria

When I was an intern in medicine at Johns Hopkins Hospital in Baltimore, Maryland, in 1968 to 1969, there was a period when a large percentage of my patients had a pleural effusion. The chief resident, Dr Richard Winterbauer, made rounds around midnight and always asked me what the thoracentesis revealed. At that time, we routinely measured the cell count and differential, glucose, and protein, and performed smears and cultures on the pleural fluid. I asked Dr Winterbauer the significance of

Why the Light criteria are still useful

The first reference to use the name the Light criteria that I am aware of was published in 1989.7 Since the original publication in 1972, there have been many studies comparing other measurements with the Light criteria for the separation of transudates and exudates, but, in general, the Light criteria have been proved to be better than anything else. I am amazed that, after 40 years, the Light criteria are still being used.

Pleural effusions have classically been divided into transudates and

Transudative effusions misclassified by the Light criteria

When the Light criteria are used, how are those transudative pleural effusions identified that are misclassified? If a patient is clinically suspected of having a transudative effusion but exudative criteria are met by a small margin (protein ratio between 0.5 and 0.65, LDH ratio between 0.6 and 1.0, pleural fluid LDH between two-thirds and the upper normal limit for serum), attempts should be made to determine whether the patient really has a transudative effusion. The 2 main measures that

Summary

The Light criteria serve as a good starting point in the separation of transudates from exudates. The Light criteria misclassify about 25% of transudates as exudates, and most of these patients are on diuretics. If a patient is thought likely to have a disease that produces a transudative pleural effusion but the Light criteria suggest an exudate by only a small margin, the serum–pleural fluid protein gradient should be examined. If this is greater than 3.1 gm/dL, the patient in probably has a

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