Lights Criteria Calculator - Pleural Fluid Transudate or Exudate

A lights criteria calculator applies the three original 1972 Light ratios to a thoracentesis sample and a serum draw to classify a pleural effusion as a transudate or an exudate, the first decision point in a pleural fluid workup.

• • Pleural fluid triage after thoracentesis: enter pleural protein, serum protein, pleural LDH, serum LDH, and the local upper normal LDH to get a transudate or exudate result with the row-level ratios.
• • Walk-through of the original 1972 paper case: recheck a parapneumonic or malignant example with all three ratios above the cut-offs to see how the three rules add up to a single classification.
• • Repeat classification after diuresis: recompute the ratios after a diuretic challenge to see whether a borderline exudate remains exudative or reverts to transudative.

The calculator is most useful when the pleural and serum samples were drawn close together and the same laboratory reported both LDH values. Visual inspection, cell count, pH, and culture still sit next to the Light's criteria in any complete pleural workup.

When the same patient also has hypoxemia on a room-air arterial blood gas, the Aa Gradient Calculator returns the alveolar to arterial oxygen gradient from the same set of arterial and gas exchange inputs and pairs naturally with the pleural workup.

The calculator computes three ratios from the five input values and applies the original 1972 OR rule, so the fluid is classified as an exudate as soon as one ratio meets its cut-off. The result panel returns the row-level ratios and a per-row positive flag.

• pleuralProtein: pleural fluid total protein in grams per deciliter, from the thoracentesis sample.
• serumProtein: serum total protein in grams per deciliter, drawn at roughly the same time as the pleural fluid.
• pleuralLDH: pleural fluid lactate dehydrogenase in units per liter, from the same thoracentesis sample.
• serumLDH: serum lactate dehydrogenase in units per liter, drawn at roughly the same time as the pleural fluid.
• upperNormalLDH: upper limit of normal for serum LDH reported by the local laboratory, often 200 U/L in adults.

The 1972 paper uses an OR across the three rows, so a fluid with only the protein ratio above 0.5 is still classified as an exudate.

The upper normal LDH is always a user-entered value so pediatric, adult, and reference laboratory ranges from 140 to 250 U/L are all respected.

According to Light et al., Annals of Internal Medicine, 1972, a pleural effusion is classified as an exudate if any of the three ratios - pleural to serum protein above 0.5, pleural to serum LDH above 0.6, or pleural LDH above two thirds of the upper normal serum LDH - is met.

According to Merck Manuals Professional, Light's criteria are the standard first step to separate transudative from exudative pleural effusions using protein and LDH ratios from a thoracentesis sample and a serum draw.

For a metabolic workup that runs in parallel with the pleural fluid review, the Anion Gap Calculator applies a similar ratio-style rule to serum electrolytes and is one of the most common lab reviews ordered alongside a thoracentesis.

Four concepts drive the result. Naming them keeps the calculator from being read as a stand-alone diagnosis and helps the next test fit the underlying cause.

Two effusions with similar protein and LDH values can still point in different directions if one patient is on diuretics, so the ratios are a starting point, not a conclusion.

Because the same protein and oncotic pressure concepts drive both the pleural fluid and the serum protein balance, the Albumin Globulin Ratio Calculator returns the serum albumin to globulin ratio from a routine chemistry panel and pairs naturally with the Light's criteria review.

The form takes five lab values from one thoracentesis and one serum draw. Each input should be entered in the same units reported by the laboratory.

1. 1 Enter the pleural fluid protein: type the pleural fluid total protein in g/dL, copied from the thoracentesis report.
2. 2 Enter the serum protein: type the serum total protein in g/dL from a blood draw taken at roughly the same time as the thoracentesis.
3. 3 Enter the pleural fluid LDH: type the pleural fluid LDH in U/L, taken from the same thoracentesis report.
4. 4 Enter the serum LDH and the local upper normal LDH: type the serum LDH in U/L and the local upper normal LDH, often 200 U/L in adult labs but different in pediatric or reference labs.
5. 5 Read the classification and the three row-level ratios: the result panel shows the transudate vs exudate classification, the count of positive ratios out of 3, the three ratios, and a one-line next-step note.

A reader who reviews a thoracentesis with pleural protein 4.6 g/dL, serum protein 6.8 g/dL, pleural LDH 250 U/L, serum LDH 200 U/L, and an upper normal LDH of 200 U/L reads protein ratio 0.68, LDH ratio 1.25, and upper normal LDH ratio 1.25, and sees 3 of 3 positive ratios and an exudate classification.

When the next-step note calls for pH review on the pleural fluid, the Arterial Blood pH Calculator returns the arterial pH, pCO2, and bicarbonate picture from the same arterial blood gas that often sits in the chart with the thoracentesis.

The Light ratios compress a small lab set into a single decision that organizes the rest of the pleural workup. Five benefits make the calculator useful at the bedside and during chart review.

• • Standardized three-row rule: the same three Light ratios are used across emergency departments, wards, and clinics, so the classification travels between providers without redefinition.
• • Row-level transparency: the result panel returns each ratio with its cut-off and a positive flag, so the contributing row is visible even when only one crosses its threshold.
• • Local upper normal LDH respected: the upper normal LDH is always a user-entered value, so the calculator adapts to local laboratory ranges without changing the rule.
• • Direct link to the next lab tests: the next-step note names pH, glucose, cell count, triglycerides, amylase, ADA, and culture as the natural follow-up for an exudate.
• • Reusable for repeat sampling: the form takes seconds to refill, which makes it easy to recompute the classification after a new thoracentesis or a diuretic challenge.

These benefits show up most clearly when the calculator is paired with a clear note about sampling time and laboratory reference range.

The ratios depend on the lab values entered and on the patient sitting in front of the calculator. Five small changes can move the classification by one row or by the full binary decision.

• • The Light's criteria are a screening rule, not a diagnosis. pH, glucose, cell count, triglycerides, amylase, ADA, and culture are needed to identify the specific exudate cause such as infection, malignancy, or chylothorax.
• • Patients on diuretics can be misclassified as exudative when the true picture is a concentrated transudate. The serum to pleural albumin gradient or NT-proBNP can clarify borderline cases without changing the Light row-level output.

According to the Merck Manuals professional edition on pleural effusion, the original Light criteria remain the standard first step, and most borderline cases come from heart-failure patients on diuretics.

According to MedlinePlus pleural effusion entry, a transudative effusion is caused by fluid leaking from increased blood vessel pressure or low blood protein, while an exudative effusion is caused by blocked vessels, inflammation, infection, lung injury, or tumors.

For a patient with chronic kidney disease or nephrotic syndrome, the GFR Calculator returns the estimated glomerular filtration rate from serum creatinine and helps explain why a transudate can show a borderline pleural to serum protein ratio in the same workup.