Low-grade fever: how to distinguish organic from non-organic forms

Low-grade fever (LGF) is defined as a body temperature between 37.5 and 38.3 degrees C, which is below the classical value reported for fever of unknown origin (FUO). We attempted to characterise its epidemiology, aetiology and clinical aspects to improve the methodological approach to diagnosis.

We reviewed and evaluated a survey of patients with LGF, followed as outpatients of our Department, a tertiary referral centre from 1997 to 2008. The same classifications were applied for classical FUO, and in the patients diagnosed with LGF, we also investigated for habitual hyperthermia (HH).

Seventy-three patients were selected and divided into two groups: group A included 32 patients classified with organic fever and group B included 41 patients with HH. Aetiology of organic LGF was: infectious disease 59%; neoplasm 3.1%; inflammatory non-infectious disease 6.2%; miscellaneous 18.7%; undiagnosed 12.5%. Mean age was significantly higher in the organic fever than in the HH group (p < 0.02). Splenomegaly and loss of weight were significantly associated with organic fever (p < 0.05), while dizziness and general malaise were associated with HH. Lack of any pathological signs at physical examination was significantly more frequent in HH (p < 0.0001). Among the biochemical tests, white blood cells and C-reactive protein were more frequently above normal limits in group A than in group B (p < 0.05).

In our experience, LGF requires the same methodological diagnostic approach as FUO, because there is no relationship between body temperature values and the severity of the underlying diseases, and the aetiological spectrum is also the same.