In the letter she highlights "increasing reports of a lack of PPE available to frontline staff" affecting staff in hospitals, GP surgeries, and care homes and for community nurses visiting people in their homes.
Theresa Fyffe said: "Our members tell us that they simply cannot obtain enough equipment, from face masks to hand sanitiser."
The RCN, she continued, welcomes news that further stocks of PPE are being distibuted but said that the RCN would be closely monitoring the situation to make sure these stocks are reaching the right places.
She also said that nursing and other staff deserve "absolute clarity" on how the government and their workplace are protecting them from the virus given discrepancies between the UK and WHO infection prevention guidance. RCN members have been reporting confusion over the guidance on what PPE to use and in what circumstances.
"We are aware that the UK NHS Pandemic Infection Prevention and Control (IPC) Guidance does not completely align with that of the World Health Organisation (WHO), specifically guidance on the use of eye protection and gowns," she said.
Theresa paid tribute to the commitment of nursing and other staff in responding to the crisis.
"Nursing staff across the country are rising to manage this unprecedented situation. Our members are coming out of retirement, students interrupting their studies, and nursing staff are deploying from non-clinical settings, all to support the front-line in the battle against Covid-19," she said. "[They] have been unstinting in their dedication and professionalism to protecting the health and wellbeing of Scotland – they must be supported by the government and health sector in return."
Theresa called on the First Minister to "to personally intervene and act to ensure enough supply of PPE and testing for Covid-19 is available for all nursing staff and our colleagues across Scotland’s health and care system".
The letter was sent also to Jeane Freeman, Cabinet Secretary for Health and Sport, and Fiona McQueen, Chief Nursing Officer for Scotland.