Patients who experience long COVID report more than 200 symptoms across 10 organ systems, according to the largest global study to date of ''long-haulers'' published on Thursday.

The researchers created a web-based survey designed to characterise the symptom profile and time course in patients with confirmed or suspected long COVID, who experience prolonged symptoms.

With responses from 3,762 eligible participants from 56 countries, the study, published in the journal EClinicalMedicine, identified a total of 203 symptoms in 10 organ systems, of which 66 symptoms were tracked for seven months.

The most common symptoms were fatigue, post-exertional malaise -- worsening of symptoms after physical or mental exertion -- and cognitive dysfunction, often called brain fog.

Of the diverse range of symptoms, others included visual hallucinations, tremors, itchy skin, changes to the menstrual cycle, sexual dysfunction, heart palpitations, bladder control issues, shingles, memory loss, blurred vision, diarrhoea, and tinnitus.

The researchers are now calling for clinical guidelines on assessing long COVID to be significantly widened beyond currently advised cardiovascular and respiratory function tests.

The assessment should include neuropsychiatric, neurological, and activity intolerance symptoms, they said.

Given the diverse make-up of symptoms that affect multiple organ systems, it is only by detecting the root cause that patients will receive the correct treatment, according the researchers.

"While there has been a lot of public discussion around long COVID, there are few systematic studies investigating this population," said Athena Akrami, a neuroscientist at University College London in the UK, and senior author of the study.

"Relatively little is known about its range of symptoms, and their progression over time, the severity, and expected clinical course (longevity), its impact on daily functioning, and expected return to baseline health," said Akrami.

The survey was open to those aged 18 or over who had experienced symptoms consistent with COVID-19, including those with and without positive SARS-CoV-2 test. It consisted of 257 questions.

In order to characterise long COVID symptoms over an extended duration, analysis of survey data was limited to respondents with illnesses lasting longer than 28 days and whose onset of symptoms occurred between December 2019 and May 2020.

Previous studies have estimated that one in seven people have some symptoms 12 weeks after a positive test result or almost 30 per cent of people 12 weeks after symptomatic disease.

In this long COVID cohort, the probability of symptoms lasting beyond 35 weeks was 91.8 per cent.

Of the 3,762 respondents, 3,608 (96 per cent) reported symptoms beyond 90 days, 2,454 (65 per cent) experienced symptoms for at least 180 days and only 233 had recovered.

In those who recovered in less than 90 days, the average number of symptoms peaked at week two, and for those who did not recover in 90 days, the average number of symptoms peaked at month two.

Respondents with symptoms over six months experienced an average of 13.8 symptoms in month seven.

During their illness, participants experienced an average of 55.9 symptoms, across an average of 9.1 organ systems.

"For the first time this study shines a light on the vast spectrum of symptoms, particularly neurological, prevalent and persistent in patients with long COVID," Akrami said.

"Memory and cognitive dysfunction, experienced by over 85 per cent of respondents, were the most pervasive and persisting neurologic symptoms, equally common across all ages, and with substantial impact on work," the scientist said.

The researchers acknowledged several limitations to the study. First, the retrospective nature of the study exposes the possibility of recall bias.

Second, as the survey was distributed in online support groups, there exists a sampling bias towards long COVID patients who joined support groups and were active participants of the groups at the time the survey was published.