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Detecting dementia using human speech is promising but faces a limited data challenge. While recent research has shown general pretrained models (e.g., BERT) can be applied to improve dementia detection, the pretrained model can hardly be fine-tuned with the available small dementia dataset as that would raise the overfitting problem. In this paper, we propose a domain-aware intermediate pretraining to enable a pretraining process using a domain-similar dataset that is selected by incorporating the knowledge from the dementia dataset. Specifically, we use pseudo-perplexity to find an effective pretraining dataset, and then propose dataset-level and sample-level domain-aware intermediate pretraining techniques. We further employ information units (IU) from previous dementia research and define an IU-pseudo-perplexity to reduce calculation complexity. We confirm the effectiveness of perplexity by showing a strong correlation between perplexity and accuracy using 9 datasets and models from the GLUE benchmark. We show that our domain-aware intermediate pretraining improves detection accuracy in almost all cases. Our results suggested that the difference in text-based perplexity values between patients with Alzheimer's Dementia and Healthy Control is still small, and the perplexity incorporating acoustic features (e.g., pause) may make the pretraining more effective.