A specimen container for separating serum or plasma (42, 57) from blood cells (40, 56) in a blood specimen (24, 54), the specimen container comprising: a specimen tube (20, 52) having an open end (22, 62), a closed end, and one or more lateral wall(s) cooperating with the closed end of the specimen tube (20, 52) to define a specimen tube interior, wherein the open end (22, 62) enables a blood specimen (24, 54) to be inserted into the specimen tube (20, 52), the specimen tube (20, 52) defining a first connection portion; a cap (30, 51) having an open end, a closed end (33, 61), and one or more lateral wall(s) cooperating with the closed end (33, 61) of the cap (30, 51) to define a cap interior, the cap (30, 51) defining a second connection portion configured to directly engage the first connection portion of the specimen tube (20, 52) to selectively secure the specimen tube (20, 52) to the cap (30, 51) with the cap interior in direct fluid communication with the specimen tube (20, 52) interior, such that when a blood specimen (24, 54) is contained within an assembled interior volume defined by the specimen tube (20, 52) and the cap (30, 51), leakage of the blood specimen (24, 54) between the specimen tube (20, 52) and cap (30, 51) is prevented; a reservoir (35, 58) which is formed when the cap (30, 51) and specimen tube (20, 52) are secured together and the capped specimen tube (20, 52) is oriented with the closed end (33, 61) of the cap (30, 51) below the closed end of the specimen tube (20, 52), characterized in that the cap (30, 51) or the specimen tube (20, 52) contains a separator (31, 55) selected from the group consisting of a thixotropic gel, a mechanical separator and a filter-based separator, the separator having a specific gravity lower than that of the blood cells and any other solids (40, 56), but higher than that of the serum or plasma (42, 57), where the volume of the reservoir (35, 58) is defined as the maximum amount of liquid that can be contained in the cap (30, 51) without coming into contact with the specimen tube (20, 52), wherein the cap with the reservoir is such that blood cells and any other solids (40, 56) are packed into the cap when the specimen tube (20, 52) is centrifuged with the capped end away from an axis of centrifugation, and the cap is configured to be removed from the specimen tube with the separator and the blood cells and other solids trapped within the cap. a specimen tube (20, 52) having an open end (22, 62), a closed end, and one or more lateral wall(s) cooperating with the closed end of the specimen tube (20, 52) to define a specimen tube interior, wherein the open end (22, 62) enables a blood specimen (24, 54) to be inserted into the specimen tube (20, 52), the specimen tube (20, 52) defining a first connection portion; a cap (30, 51) having an open end, a closed end (33, 61), and one or more lateral wall(s) cooperating with the closed end (33, 61) of the cap (30, 51) to define a cap interior, the cap (30, 51) defining a second connection portion configured to directly engage the first connection portion of the specimen tube (20, 52) to selectively secure the specimen tube (20, 52) to the cap (30, 51) with the cap interior in direct fluid communication with the specimen tube (20, 52) interior, such that when a blood specimen (24, 54) is contained within an assembled interior volume defined by the specimen tube (20, 52) and the cap (30, 51), leakage of the blood specimen (24, 54) between the specimen tube (20, 52) and cap (30, 51) is prevented; a reservoir (35, 58) which is formed when the cap (30, 51) and specimen tube (20, 52) are secured together and the capped specimen tube (20, 52) is oriented with the closed end (33, 61) of the cap (30, 51) below the closed end of the specimen tube (20, 52), characterized in that the cap (30, 51) or the specimen tube (20, 52) contains a separator (31, 55) selected from the group consisting of a thixotropic gel, a mechanical separator and a filter-based separator, the separator having a specific gravity lower than that of the blood cells and any other solids (40, 56), but higher than that of the serum or plasma (42, 57), where the volume of the reservoir (35, 58) is defined as the maximum amount of liquid that can be contained in the cap (30, 51) without coming into contact with the specimen tube (20, 52), wherein the cap with the reservoir is such that blood cells and any other solids (40, 56) are packed into the cap when the specimen tube (20, 52) is centrifuged with the capped end away from an axis of centrifugation, and the cap is configured to be removed from the specimen tube with the separator and the blood cells and other solids trapped within the cap.