When discussing side-effects of chemotherapy, you may hear the word “nadir”, mostly in relation to the blood counts such as white blood cell count and platelet count.
Nadir influences White and Red Blood Cell Counts and Platelet Count
When chemotherapy is administered, healthy cells get affected along with the rapidly multiplying cancer cells. Certain healthy cells, especially those that divide at a fast rate are susceptible to damage. These can be found in the hair, the cells lining the mouth and the intestinal tract, and the blood cells (white and red blood cells and also platelets).
Blood cells are produced in the bone marrow – the soft, spongy matter found inside larger bones in the body. Basic building blocks known as stem cells are responsible for the production of all types of blood cells. Stem cells do not divide quickly and they are less likely to get damaged by chemotherapy. As cells mature, there are certain stages when they reproduce at a faster rate. This is when they are more susceptible to chemotherapy. Cells that are more mature can continue to develop as fully mature cells for many days after chemotherapy is administered. When these cells die, the circulating supply reduces significantly and the blood counts reach a low point, referred to as the nadir.
Returning to Normal after a Nadir
In 3 to 4 weeks, the blood counts will reach normal levels, after the body’s response system has signaled the stem cells inside the bone marrow to start producing new cells and increase production. If chemotherapy is administered at a time when the stem cells are in the process of increasing their production levels, it could inflict permanent damage to bone marrow cells. These factors are considered when planning chemotherapy treatment cycles. For instance, some drugs are administered on the first and the eighth day of a 28-day cycle. Administered a week after the first, the second dose of chemotherapy is tolerated because at this time, the stem cells have yet to increase their production (or they are still at nadir level). Production has not been increased because the second dose of chemotherapy is administered prior to the time when the blood counts have reached their nadir.
Although nadir is usually reached in 10 days after treatment, it can vary depending on the administered drugs. The main concern associated with the nadir period is that the body’s foremost defense mechanism against infection, i.e. the white blood cells and the platelets that enable the blood to clot, are both making the patient more vulnerable to infection and bleeding. The subsequent dose is administered only after the blood counts of the patient have progressed from the nadir and reached safe levels.