We experimentally demonstrate an all-silicon nanoantenna-based micro-optofluidic cytometer showing a combination of high signal-to-noise ratio (SNR) > 14 dB and ultra-compact size. Thanks to the ultra-high directivity of the antennas (>150), which enables a state-of-the-art sub-micron resolution, we are able to avoid the use of the bulky devices typically employed to collimate light on chip (such as lenses or fibers). The nm-scale antenna cross section allows a dramatic reduction of the optical system footprint, from the mm-scale of previous approaches to a few µm2, yielding a notable reduction in the fabrication costs. This scheme paves the way to ultra-compact lab-on-a-chip devices that may enable new applications with potential impact on all branches of biological and health science.