Participants without MGUS, matched to MGUS participants by age and gender.

Participants receive a Mental Health and Quality of Life questionnaire that will be answered electronically on the study website (www.blodskimun.is)

Participants previously diagnosed as having MGUS or participants that have multiple myeloma or other lymphoproliferative diseases will receive standard clinical follow-up and treatment.

Standard clinical follow-up according to International Myeloma Working Group (IMWG) guidelines.

Intensive clinical follow-up with regular clinical assessment, laboratory testing and radiology imaging.

Control clinical visit

Satisfaction with life scale (SWLS) is administered at baseline and during follow-up to assess global satisfaction with one's life according to the respondent ́s personal values and criteria, and therefore, is free of the researchers' personal values and criteria of specific areas of life deemed as important. SWLS is a five item self-report scale used to measure global life satisfaction. The items are scored on a 7-point scale, ranging from one (strongly disagree) to seven (strongly agree). A higher score reflects a greater satisfaction with life. Scores within 0-4 indicate minimal, 5-9 mild, 10-14 indicate moderate, 15-19 moderately, and 20-27 severe depressive symptoms. An optimal clinical cut off point for a Major Depressive Disorder (MDD) diagnosis is estimated to be a score of 12 or greater.

The happiness question is administered at baseline and during follow-up to assess happiness. A single item measure for happiness is one question measure that assesses happiness in an individual's life. Happiness is estimated by an individual's response to the question: "By taking into account all aspects of your life, how happy do you consider yourself?" that is scored on a 10-point scale ranging from one (very unhappy) to ten (very happy). Higher scores indicate more happiness.

The Connor-Davidson resilience scale (CD-RISC) is administered once during the study period to assess and control for resilience. The CD-RISC is a 10-item self-report measure (i.e., the short form of the questionnaire) assessing resilience in the past month. Each item is assessed on a five-point scale ranging from zero to four. Scores on each item are summed, and therefore, the total scores range from zero to 40. Higher scores indicate more resilience.

Multidimensional scale of perceived social support (MSPSS) is administered once during the study period to assess and control for social support. The MSPSS is a 12-item self-report questionnaire addressing relationships with family, friends, and significant other in the areas of social popularity, respect, and areas directly related to social support. Each item is assessed on a six-point scale ranging from one (Very strongly disagree) to seven (Very strongly agree). Score on each item is summed and averaged to both calculate the total scores and scale scores. Higher scores indicate more social support. Mean scores ranging from 1 to 2.9 can be considered low support, a score of 3 to 5 can be considered moderate support, and a score from 5.1 to 7 can be considered high support.

Adverse childhood experiences international questionnaire (ACE-IQ) is administered once during the study period to assess and control for adverse events in childhood. The ACE-IQ is 43-item self-report measure assessing adverse childhood experiences. Questions cover family dysfunction, physical, sexual and emotional abuse and neglect by parents or caregivers, peer violence, witnessing community violence, and exposure to collective violence. Some items are yes or no questions, while other items are assessed on a five-point scale ranging from one (Always) to five (Never) or a four-point scale ranging from one (Many times) to four (Never).

Patient health questionnaire (PHQ-9) is administered at baseline and during follow-up to assess depressive symptoms. The PHQ-9 is a nine item self-report questionnaire that measures frequency of depressive symptoms in the past two weeks. Each item is scored on a 4-point scale ranging from zero (none at all) to three (nearly every day). Therefore, overall scores range from zero to 27, with a higher score indicating more severe depressive symptoms. Scores between 0-4 indicate minimal depressive symptoms, 5-9 indicate mild depressive symptoms, 10-14 indicate moderate depressive symptoms, 15-19 indicate moderately severe depressive symptoms, and 20-27 indicate severe depressive. In addition, an optimal clinical cut off point for a MDD diagnosis is estimated to be a score of 12 or greater.

Generalized anxiety disorder (GAD-7) is administered at baseline and during follow-up to assess anxiety symptoms. The GAD-7 is a self-report measure that assesses frequency of anxiety symptoms in the past two weeks, which in part correspond to the diagnostic criteria of generalized anxiety disorder in the DSM-IV. The questionnaire consists of seven items that assess how often in the past two weeks respondents experienced various symptoms of anxiety (e.g., feeling nervous, anxious or on edge). Each item is assessed on a 4-point scale that ranges from zero (not at all) to three (nearly every day). Total scores therefore range from zero to 21, with a higher score indicating more severe anxiety symptoms. Scores between 0-4 indicate minimal anxiety symptoms, 5-9 mild anxiety symptoms, 10-14 moderate anxiety symptoms, and 15-21 severe anxiety symptoms (Spitzer et al., 2006). In addition, an optimal clinical cut off point for a GAD diagnosis is estimated to be a score of 10 or greater

The worry question is administered at baseline and during follow-up to assess worry thoughts. The single item measure for worry is a single item measure for health-related worry. Worry is estimated by an individual's response to the question: "How much do you worry about your health?" that is scored on a 10-point scale ranging from one (very little) to ten (very much). Higher scores indicate more worry.

Perceived stress scale (PSS-10) is administered during follow-up. The PSS-10 is a 10-item self-report measure on the degree in which life situations are appraised as stressful (i.e., unpredictable, uncontrollable, and overloading). PSS-10 is a 10-item self-report measure that assess the degree to which life situations were appraised as stressful in the past month, in particular how unpredictable, uncontrollable, and overloading individuals find their lives. Each item measures the frequency on a 5-point scale that ranges from 0 (never) to 4 (very often). The total score is calculated by reversing scores on four positively phrased items, and thereafter, summing the scores on each item. Total scores range from 0 to 40. Scores between 0-13 indicate low perceived stress, 14-26 moderate perceived stress, and 27-40 high perceived stress.

Posttraumatic stress disorder checklist for DMS-5 (PCL-5) including the Life Events Checklist for DSM-5 (LEC-5) is administered during follow-up. The PCL-5 is a 20-item self-report measure that assesses the severity of PTSD symptoms in the past month. Each item assesses a symptom of PTSD. The items on the PCL-5 measure how much an individual has been bothered by each symptom associated with a particular stressful event on a five-point scale ranging from zero (not at all) to four (extremely). Total scores range from 0-80 and higher scores indicate more severe symptoms of PTSD. According to previous studies a score of 33 or higher indicates the presence of clinically significant PTSD symptoms. The LEC is a self-report instrument that screens for 16 lifetime traumatic events that are known to potentially result in clinically significant distress including PTSD, in addition, other stressful events not captured by the 16 items can be reported

36-Item short form survey (SF-36) is administered during follow-up. The SF-36 is a 36-item questionnaire assessing physical functioning, bodily pain, role limitations due to physical health and personal or emotional problems, general psychiatric health, social functioning, energy and fatigue, and general health perceptions. The measure also includes a single item that provides an indication of perceived change in health. Each item is assessed on a six-point scale ranging from one to six, which are recoded per a scoring key so that values range from zero to 100. Scores indicate the percentage of total possible score achieved. All items are recoded so that a high score indicates a more favorable health state. Thereafter, the average score is calculated for each scale. Items that are left blank (missing data) are not considered when calculating the scale scores.