Light's criteria false exudates in heart failure

What a false exudate is under Light's criteria

A false exudate, also called a pseudoexudate, is a transudative pleural effusion that Light's criteria label as an exudate. Light's criteria, published by Light and colleagues in 1972, call an effusion an exudate if the pleural fluid to serum protein ratio is above 0.5, the pleural fluid to serum LDH ratio is above 0.6, or the pleural fluid LDH is above two-thirds of the upper limit of normal for serum LDH. Because a single positive test is enough, the rule favours sensitivity over specificity. Porcel's 2013 review states that the criteria identify 98% of exudates but misclassify about 25% of transudates, and Light's own 2013 review adds that most of these patients are on diuretics. In Bielsa's 2012 series of 364 heart failure effusions and 102 hepatic hydrothoraces, Light's criteria mislabelled 29% of the heart failure effusions and 18% of the hepatic ones as exudates, a significant difference.

Why diuretics turn heart failure transudates into false exudates

Diuretic treatment raises the concentration of protein and LDH in a heart failure effusion, which can carry it over Light's cut-offs. Chakko and colleagues sampled the effusions of eight patients with heart failure before treatment and again about six days later, after diuresis. The mean pleural protein rose from 2.2 to 3.2 g/dL and pleural LDH from 116 to 183 U/L; the mean protein ratio went from 0.34 to 0.47 and the LDH ratio from 0.39 to 0.64, past the 0.6 cut-off. Three effusions that started as transudates met the exudate criteria after treatment. Romero-Candeira and colleagues repeated thoracentesis in 21 heart failure patients until the effusions cleared: pleural protein rose from 23 to 33 g/L, albumin from 13 to 18 g/L and LDH from 177 to 288 U/L. The serum minus pleural fluid gradients for albumin and protein rose far less, by 12% and 11%, against about 47% and 48% for the pleural concentrations.

That difference is why the gradients, rather than the ratios, are used to recheck a heart failure effusion after diuresis. Red cells matter too: in Eid's 2002 review of 175 heart failure patients who had a thoracentesis, most exudates had a noncardiac cause, and 4 of the 12 exudates explained by heart failure alone could be put down to red cell contamination of the fluid, which raises LDH.

The albumin and protein gradients that correct Light's criteria

The serum minus pleural fluid albumin gradient and protein gradient are the two published checks for a false exudate. Each is simply the serum value minus the pleural fluid value, measured at about the same time. Roth and colleagues introduced the albumin gradient in 1990: in 59 patients, a gradient of 1.2 g/dL or less to mark exudates classified 57 correctly. Light's criteria caught every exudate in that study but labelled five heart failure patients as exudates, four of whom had already had diuretics. Bielsa's 2012 study then compared the two gradients on the effusions that Light's criteria had mislabelled. An albumin gradient above 1.2 g/dL correctly identified 83% of the heart failure false exudates and 62% of the hepatic ones, while a protein gradient above 3.1 g/dL identified 55% and 61%. A pleural fluid to serum albumin ratio below 0.6 identified 78% and 77%. Porcel's review concludes that the albumin gradient is the simplest test for heart failure.

The gradients are a second step, not a replacement. Romero-Candeira's 2002 study of 249 patients found Light's criteria 93% accurate overall, better than the albumin gradient (87%) or protein gradient (86%) alone. In the 72 patients on diuretics, Light's criteria fell to 83%, similar to the albumin (88%) and protein (86%) gradients, and applying the gradients only to diuretic-treated patients with a clinical transudate but a Light's exudate did not improve the result significantly. Light's 2013 advice is to check the gradient when a transudate is likely clinically and the criteria suggest an exudate by only a small margin.

Worked example: a borderline exudate in heart failure

This worked example shows how a heart failure effusion can meet Light's criteria by a single, narrow margin and how the gradients settle it; the numbers are illustrative and hand-checked. A patient with known heart failure has had diuretics for several days when a unilateral effusion is tapped. Pleural fluid protein is 3.4 g/dL and serum protein 6.6 g/dL, a ratio of 0.52, just above 0.5. Pleural LDH is 140 U/L and serum LDH 260 U/L, a ratio of 0.54, below 0.6. The laboratory's upper limit for serum LDH is 250 U/L, so two-thirds is 166.7 U/L, and 140 does not exceed it. One criterion is met, so Light's criteria call the effusion an exudate. The protein gradient is 6.6 minus 3.4, or 3.2 g/dL, above 3.1. Serum albumin is 3.4 g/dL and pleural albumin 1.9 g/dL, a gradient of 1.5 g/dL, above 1.2. Both gradients point to a transudate.

For contrast, a patient with pneumonia and an effusion has pleural protein 4.6 g/dL against serum 6.8 g/dL (ratio 0.68), and pleural LDH 520 U/L against serum 240 U/L (ratio 2.17) with the same upper limit of 250 U/L. All three criteria are met by a wide margin, and the protein gradient of 2.2 g/dL and albumin gradient of 0.6 g/dL (serum 3.0, pleural 2.4) do not suggest a transudate. Enter both patients in the Light's criteria calculator to see each ratio and the protein gradient it reports for every exudate.