The Hb-mass test measures the total amount of hemoglobin (expressed in in grams) and the total volume of blood in the body of the athlete. 

This is an indirect and invasive measurement (it requires multiple withdrawals of blood) based on the dilution of a known amount of a substance in the blood of the individual; this substance is a gas given by inhalation: CARBON MONOXIDE (CO). 

CO is a poisonous gas that links to hemoglobin with an affinity that is more than 200 times higher than oxygen, determining a situation of hypoxia, reducing both the transport in the blood and the peripheral transfer of O2 (the dissociation curve of Hb moves to the left), as well as a direct toxic effect on cytochrome-oxidase in mitochondria. 

The inhaled CO binds tenaciously to hemoglobin, forming carboxyhemoglobin (COHb): COHb concentrations of less than 5% are normal in population, in smokers it comes up to 7-10%, in cases of CO poisoning when the COHb exceeds 20-30% treatment in a hyperbaric chamber has to be taken to avoid serious consequences to the brain and the heart. Higher concentrations can be fatal. 


The test protocol proposed to athletes provides the inhalation of a "bolus" of CO in quantities between 50 and 100 ml (depending on the subject) in order to elevate his/her COHb by 5-7%: this means that if the baseline COHb level is 3%, with the test it increases to about 10%, which is equal to 1/2 - 1/3 of the concentration hazardous to health. 
The elimination of inhaled CO is very slow and results amongst other things to a reduction of VO2max and aerobic performances by 3-5% for 1-3 days (Eur J Appl Physiol 2005, 95:486-495). 

The test is not easy to perform and requires the presence of experienced personnel in order to limit errors; notwithstanding this, successive measurements within a few days may show differences of up to 15% (J Appl Physiol 2005, 99: 1745-1758). 
The inter-subject variability (within subject CV) of Hb-mass measured with this method indicates that you can have variations of up to 20%, remaining inside the 1/1000 probability proposed by WADA for the biological passport (Med Sci Sports Exercice Sep 2011 24). 
As endurance athletes have 8-10 liters of blood, it is evident that the method can identify with certainty only blood manipulations superior to 1.5-2 liters. 

Although this test is essentially useless and potentially «dangerous to health», the UCI strongly «recommends» teams to perform these tests on their athletes at affiliated Institutions. Some cyclists have been forced by teams and the UCI to repeated measurements, otherwise they would see the cancellation of the contract or the opening of disciplinary proceedings. 
Could it be a sort of «license of virginity» to be displayed in the scope of the fight against doping?