Red blood cells (RBC)Normal for men: 4.5 to 6.2 million per microliter (a single drop). Normal for women: 4.2 to 5.4 million per microliter.
White blood cellsNormal for men and women: 3,700 to 10,000 per microliter. Lowest level at which someone is safe from infection: 1,000.
PlateletsNormal range for men and women: 150,000 to 450,000 per microliterLowest level at which someone's blood can still clot normally: 100,000. Level at which there's a risk of spontaneous bleeding: 50,000. Level at which bleeding can become life-threatening: 5,000.

Hemoglobin, the protein in red blood cells that oxygenates the blood, provides the body with energy, strength, and stamina. When there isn't enough oxygen in the blood, he'll feel tired, weak, and sometimes faint or dizzy. He may have trouble catching his breath or feel pain in his chest as he tries to breathe. Extreme fatigue can bring with it memory or concentration problems, so he might seem confused or act like he's not thinking straight. For more information, see Understanding Low Red Blood Counts. Platelets control clotting and stop bleeding, so when his platelets are low, he bruises easily and his body has trouble stopping the bleeding from even a small cut or scratch. (Women who are still having periods may have unusually heavy flow.) Nosebleeds and bleeding gums after brushing teeth can become an ongoing problem. Some people also get a rash that looks like little pinprick-sized red or purple dots, or they experience aching in their joints or muscles. Headaches are common too. For more information, see Understanding Low Platelets. The technical name for low white blood cell count is neutropenia, from the word neutrophil, and it means that his immune system is depressed. Another term you may hear is immunocompromised. Without regular immune function, his body can't fight off germs and infections as it normally would. Watch closely for signs of fever, sore throat or cough, or chills and shaking. Keep an eye out for other signs of infection such as an injury that gets red and pus-filled or doesn't heal. Another sign is diarrhea or loose stools for more than two days in a row. On the opposite extreme, an overly high white blood count is a sign his body is fighting an infection, although those fighting leukemia also can have elevated white counts.

Because the ranges considered normal are very wide, interpretation of the results can get muddy. One patient with a platelet count of 100,000 might have only minor bruising, for example, while another might have constant nosebleeds. It's also important to keep in mind that a patient's blood counts that are low as a result of chemotherapy and will rise again as his body manufactures new cells. Most of the time, it's simply a case of waiting for the counts to rise again and keeping him safe in the meantime. However, for each count there is a level below which his doctor will take steps to elevate blood counts. These may include prescribing medicines or giving transfusions. Likewise, don't be surprised if his doctor postpones a chemo treatment if she decides his blood counts are too low