**7. Diagnosis**

For a diagnosis of TBE, three criteria should be fulfilled:


#### **7.1. Blood and cerebrospinal fluid analysis**

In the initial (viremic) phase of TBE leukopenia and/or thrombocytopenia are ascertained in around 70% of patients, and abnormal liver test results are seen in about 20% [78]. In the second (meningoencphalitic) phase, platelet count is normal, whereas peripheral blood leukocyte count is normal or mildly elevated (rarely >15 × 109 /L). Concentration of C‐reactive protein and erythrocyte sedimentation rate is usually in normal range throughout the entire course of TBE. In the initial phase of TBE, CSF findings are in the normal range, whereas in the second (meningoencephalitic) phase, elevated CSF leukocyte counts (usually <500 × 10<sup>6</sup> /L, extremely rarely ≥1000 × 10<sup>6</sup> /L), a normal to moderately elevated protein concentration, and a normal glucose concentration are present. A typical finding is lymphocytic pleocytosis; however, in the first few days of the meningoencephalitic phase of TBE, neutrophils may predominate in CSF. Elevated lymphocyte counts may persist for several weeks after clinical recovery [32, 79].

#### **7.2. Magnetic resonance imaging (MRI) abnormalities**

MRI abnormalities of brain and spinal cord are present only in about 20% of patients with TBE. According to a study performed by Kaiser [46], they are found more often in patients with meningoencephalomyelitis (7/25, 29%) than in patients with meningoencephalitis (11/64, 17%), and are not seen in those with meningitis (0/13). Increased signal intensity is most often seen in the thalamus, but can (also) be present in basal ganglia, internal capsule, splenium, cerebellum, peduncles and brain stem [46, 80–92]; in patients with myelitis, the abnormalities are seen predominantly in the anterior horns of the spinal cord [84, 85, 89, 93–97]. Studies of specificity are lacking but the specificity is probably low [89].
