The goal of this observational study is to describe a new disease condition of histiocytosis related to injecting drug use, its preconditions, symptoms, signs, findings, and prognosis in a detailed and systematic patient series in one referral center. The main question[s] it aims to answer are:

• to find specific histologic features in different tissues to help the differential diagnosis from other histiocytoses
• to describe the value of chitotriosidase activity to screen this condition
• to describe the value of various tissue biopsies in confirming the diagnosis
• to describe the nature of polyvinylpyrrolidone accumulation in tissue macrophages by novel special microscopic techniques
• to find new tandem mass spectrometry methodology to prove polyvinylpyrrolidone accumulation in macrophages
• to show that polyvinylpyrrolidone treatment activates macrophages to histiocytes and causes povidone accumulation within the cells in in vitro experiments
• to evaluate the pathology of macrophage activation to histiocytes by transcriptomics

The patient history will be collected from the data produced by follow-up of cases followed up in a single center. A subgroup of participants will be given an opportunity to sign informed consent to give access to/donate blood and tissue samples to search for techniques to prove polyvinylpyrrolidone storage within histiocytes, and to search for transcriptomics signal(s) in histiocytes.

Researchers will compare blood and tissue samples from the biobank as controls.

Comprehensive patient data (disease history, findings, complications, laboratory findings, histopathology, and imaging) will be obtained from patient records of a single university hospital. Coded histiocytosis-positive tissue samples will be compared to biobank samples from corresponding tissues from healthy and other histiocytosis tissue samples to show polyvinylpyrrolidone storage with novel methodologies.

In addition, if a study participant who is followed up clinically, signs informed consent, monocytes are separated from a blood sample, differentiated to macrophages, and exposed to polyvinylpyrrolidone. Various novel methods will be utilized to explore polyvinylpyrrolidone within histiocytes.

Inclusion Criteria:

• All identified adult patients in our records with a history of injecting drug use by dissolving tablets containing polyvinylpyrrolidone and (suspected) PVP-histiocytosis

Exclusion Criteria:

• Other histiocytosis diagnosed by tissue samples and clinical information

Adult patients admitting to intravenous use of dissolved tablets (indicated for oral use) containing polyvinylpyrrolidone (PVP30) and positive tissue sample in any tissue biopsy compatible with features of histiocytosis caused by PVP30

Study data can only be acquired by proper inquiry to the national authority Findata by law