Many carriers of the potentially lethal Clostridium difficile bug are missed by unreliable tests, researchers say.
Analysis of 18 studies by St George's, University of London, found one test had wrongly given the all-clear to a quarter of those infected.
The variation in performance between the six tests studied could mean misleading infection rate comparisons between different hospitals.
The study was published in the Lancet Infectious Diseases journal.
While deaths related to the "superbug" MRSA have shown falls in recent years, those related to Clostridium difficile are rising fast.
The bug is found naturally in the gut of approximately 3% of adults, and presents no threat as long as the normal balance of gut bacteria is maintained.
However, in weak or frail patients, particularly those on antibiotics, which can disrupt this balance, the bacterium can cause diarrhoea and severe inflammation of the bowel, which can be fatal.
Quick and accurate diagnosis is key, so that patients can be isolated, and other measures taken to try to control the spread of the bug to other vulnerable people.
Currently, a single test is used on a stool sample from the patient, looking for higher levels of a toxin produced by the active bacteria.
However, the London analysis suggests this may not be enough to find the right patients.
While most tests produce a small proportion of "false positives" and "false negatives", the percentage of cases missed ranged from 5% to just over 24.3%, and the percentage of "false positives" ranged from 3% to 45%.
Dr Timothy Planche, a clinician at St George's Healthcare NHS Trust, who led the study, said: "A false negative result could mean that infected patients don't get the right treatment and could pass the infection on to others.
"Conversely, patients receiving a false positive result may receive inappropriate treatment and be placed in wards along with infected patients, putting them at risk of contracting the infection."
He recommends improving the performance of the tests by using a second test to check the first one.
Most hospitals would tend to rely on just one variety of test, and this could skew any comparisons of C. diff rates between trusts.
Professor Richard James, from Nottingham University, said the "deficiencies" of current tests were a "serious problem".
He added: "Rapid molecular tests have recently been introduced for MRSA and will become available for C. difficile shortly, but they are more costly than the current tests used in the NHS.
"NHS microbiology laboratories have been under-resourced for many years, and will find it difficult to perform these more rapid tests unless the NHS takes a more holistic economic view that the benefits in reducing infections to both the NHS and society justify the extra costs of more rapid tests for serious infections such as C.difficile and MRSA."