Hubungan kadar awal D-dimer dengan tingkat keparahan penyakit serta lama rawat pada pasien COVID-19 derajat sedang, berat, dan kritis di RSUP Persahabatan pada Januari - Desember 2021 = The Relationship between initial D-dimer levels and disease severity as well as length of hospital stay in moderate, severe, and critical COVID-19 patients at RSUP Persahabatan from January to December 2021