Knee extension test in sitting

Testing if a patient can actively extend both knees and achieve at least a 3/5 grade knee extensor strength on Oxford scale is a basic way of assessing their capacity to attempt a sit to stand or gait.

Causes for low HGB

The most obvious cause for an acute reduction in haemoglobin is haemorrhage! i.e. bleeding

(I'm writing this because I was asked today and stupidly searched for some kind of complex cause and in doing so failed to come up with the most likely cause!) 

This could be internal bleeding e.g. peptic ulcer or inflammatory bowel disease as well as external bleeding from a haemorrhage

Hemolysis 

Kidney disease 

One of the functions of the kidney is to produce a hormone called erythropoeitin which signals the bone marrow to produce red blood cells. Kidney disease can result in reduced levels of this hormone causing chronic anemia (according to livestrong) 

Chemotherapy, blood toxicity, immunosuppresants

Aplastic anemia 

Hepatic failure 

Vitamin B12 deficiency 

Cardiac failure