A variable combination of laboratory findings of TIN, such as rise in the plasma creatinine concentration, eosinophilia, eosinophiluria, changes of urine sediment, as well as evidences of tubulointerstitial damage based upon the culprit agent may present.

In some studies a variable amount of proteinuria among older patients and NSAID-induced AIN has been reported, although nephrotic-range proteinuria among patients with TIN is rare.

Laboratory findings consistent with the diagnosis of TIN include:

• Increased plasma creatinine
• Eosinophilia and eosinophiluria
• Changes of urine sediment
• Evidence of tubulointerstitial damage
• Variable amount of proteinuria

About 23% of patients have eosinophilia. Eosinophilia, when present, can be helpful in the evaluation of tubulointerstitial nephritis. However, this finding is neither specific nor sensitive enough to establish the diagnosis. Although the true incidence of eosinophilia in acute tubulointerstitial nephritis is unknown, it is estimated to be present in approximately half of patients. Typically, eosinophilia is absent in acute tubulointerstitial nephritis that is induced by nonsteroidal anti-inflammatory drugs (NSAIDs).

Urinary findings include:

• Eosinophiluria: sensitivity is 67% and specificity is 83%.^[1] The sensitivity is higher in patients with interstitial nephritis induced by methicillin or when the Hansel’s stain is used.
• Isosthenuria ^[2]