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1.Early/acute/serous stage, one or more of:
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•pH < 7.2
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•glucose < 40 mg/dL
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•LDH >1000IU/dL
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•protein > 2.5 g/dL
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•WBC > 500/µL
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•specific gravity greater than 1.018
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•Thin serous or cloudy fluid, generally sterile
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2.Fibrinopurulent/intermediate stage
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•Thicker, opaque fluid or fluid with positive cultures
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3.Organizing/late stage
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•An organizing peel with entrapment of the lung