Hertfordshire GP Dr Mike Ingram said GPs in his area can request eight tests from each blood sample from the local laboratory.
'The reason is clearly to limit the number of blood tests. It is ludicrous. We will now have to select which ones we don't want to do. What happens if I don't order a calcium test and the patient has a calcium disorder?'
Dr Ingram said he would now have to make several requests, use paper forms or send samples to another laboratory.
A spokeswoman for West Hertfordshire Hospitals NHS Trust, which runs the laboratory, said a new IT system had been installed to improve access to test results.