The purpose of a thick blood film in malaria diagnosis is to detect low-density parasitemia. A thick film concentrates a larger volume of blood, allowing for the visualization of even a small number of parasites. This increased sensitivity is crucial in cases where the parasite load is low and may be missed on a thin film. While thin films are used to identify species and assess parasite morphology, thick films are the preferred method for initial detection due to their enhanced sensitivity. Thus, thick films are essential for accurate diagnosis, especially in early or low-grade infections.

Reference: Medical Microbiology, Murray et al., 8th Edition.