***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: Venous thromboembolism is a serious complication following gynecologic surgery and is particularly common in patients with malignancy . A previous study of subcutaneous low-dose heparin given as one dose preoperatively and every 12 hours postoperatively failed to show a benefit in gynecologic oncology patients . In the present study , two more intense regimens of low-dose heparin were evaluated . Three hundred four patients were assigned r and omly to receive no prophylaxis ( controls ) , subcutaneous heparin 5000 units 2 hours before surgery and every 8 hours postoperatively ( low-dose heparin ) ( regimen I ) , or 5000 units heparin subcutaneously every 8 hours preoperatively ( between two and nine doses ) and every 8 hours postoperatively ( regimen II ) . All patients had thromboembolism surveillance with the fibrinogen uptake test and clinical evaluation . Eighty-four percent had a malignancy . Thromboemboli were diagnosed in 19 of 103 control patients , ten of 104 regimen I patients , and six of 97 regimen II patients , a statistically significant difference ( P < .008 ) . When compared with the control group , the study groups had no evidence of increased bleeding complications or alteration of laboratory coagulation indicators . ( Obstet Gynecol 75 : 684 , 1990 In a prospect i ve double-blind trial , low molecular weight ( LMW ) heparin ( KABI 2165 ) 5,000 U ( anti-Xa ) once daily was compared with conventional heparin 5,000 IU twice daily , both given subcutaneously , as regards prevention of postoperative deep venous thrombosis ( DVT ) in 52 patients undergoing major abdominal surgery . Radioactive fibrinogen uptake test ( FUT ) was used for DVT screening . DVT , diagnosed from positive FUT , developed in two patients from each group , but could be phlebographically confirmed in only one ( LMW ) case . No intergroup differences were found in peroperative blood loss or requirements for blood transfusion . Complications attributable to the prophylactic regimens were few . In the LMW-heparin group , the anti-Xa levels measured during operation showed considerable variation , the higher activities ( greater than 0.30 U/ml ) being nonsignificantly associated with increased blood loss . Studies with lower doses of LMW-heparin are recommended PURPOSE : Routine deep venous thrombosis prophylaxis is controversial in Asian patients , because deep venous thrombosis incidence was considered negligible . Because of recent reports of significantly higher incidences , a r and omized , controlled trial was conducted to assess the effectiveness and complications of enoxaparin prophylaxis ( low molecular weight heparins ) in major colorectal surgery . METHODS : Three hundred twenty consecutive patients were r and omly assigned to control or low molecular weight heparins groups . Patients in the low molecular weight heparins group were given perioperative enoxaparin starting 12 hours before surgery . The surgeon ( blinded ) assessed for difficulties related to possible enoxaparin administration . Independent blinded observers performed daily clinical assessment s and Doppler studies ( at the 3rd and 5th postoperative day ) . Deep venous thrombosis was confirmed by duplex ultrasound , and pulmonary embolism was confirmed by lung scans or postmortem examinations . RESULTS : Deep venous thrombosis developed in 5 of 169 ( 3 percent ) controls and 0 of 134 low molecular weight heparins patients ( P=0.045 ) . Three of the deep venous thrombosis patients had pulmonary embolism , which was fatal in one patient . The surgeons were unable to perceive any increased surgical difficulties in the low molecular weight heparins group . The bleeding-related complications were significantly higher in the low molecular weight heparins patients ( controls , n=3 ( 1.8 percent ) ; low molecular weight heparins , n=9 ( 6.7 percent ) ) . However , apart from one subdural hematoma and two abdominal hemorrhages needing re-exploration , which also occurred in one of the controls , these complications were minor bruises at the wounds , drains , or injection sites . CONCLUSION : Deep venous thrombosis prophylaxis is needed in Asian patients undergoing major colorectal surgery Summary : A r and omized controlled trial was undertaken comparing the efficacy and safety of low molecular weight ( LMW ) heparin ( Fragmin ) with sodium heparin for prophylaxis against postoperative thromboembolic disease after major gynaecological surgery . Women were r and omized to receive subcutaneous injections of 5,000 U of either once daily LMW heparin or twice daily sodium heparin . A total of 566 women were recruited , of whom 552 completed the study . Most women ( 461 ) had malignant disease and 430 of these underwent radical surgery . The remainder underwent major , but not radical surgery . There were 5 thromboembolic events in the LMW heparin group and 2 in the sodium heparin group , with no significant difference between these groups . No significant difference was found in the incidence of intraoperative or postoperative transfusion in the 2 groups . The decision of which heparin to use in routine practice can not be made on clinical grounds BACKGROUND Unfractionated heparin and the low molecular weight heparin , dalteparin , are used for prophylaxis against venous thromboembolism in patients undergoing craniotomy . These drugs were compared in a r and omized , prospect i ve pilot study comparing intermittent pneumatic compression devices plus dalteparin to intermittent pneumatic compression devices plus heparin . METHODS One hundred patients undergoing craniotomy were r and omly allocated to receive perioperative prophylaxis with subcutaneous ( SC heparin , 5000 units every 12 hours , or dalteparin , 2,500 units once a day , begun at induction of anesthesia and continued for 7 days or until the patient was ambulating . Entry criteria were age over 18 years , no deep vein thrombosis ( DVT ) preoperatively as judged by lower limb duplex ultrasound and no clinical evidence of pulmonary embolism preoperatively . Patients with hypersensitivity to heparin , penetrating head injury or who refused informed consent were excluded . Patients underwent a duplex study 1 week after surgery and 1 month clinical follow-up . All patients were treated with lower limb intermittent pneumatic compression devices . RESULTS There were no differences between groups in age , gender , and risk factors for venous thromboembolism . There were no differences between groups in intraoperative blood loss , transfusion requirements or postoperative platelet counts . Two patients receiving dalteparin developed DVT ( one symptomatic and one asymptomatic ) . No patient treated with heparin developed DVT and no patient in either group developed pulmonary embolism . There were two hemorrhages that did not require repeat craniotomy in patients receiving dalteparin and one that did require surgical evacuation in a patient treated with heparin . Drug was stopped in two patients treated with dalteparin because of thrombocytopenia . None of these differences were statistically significant . CONCLUSION There was no significant difference in postoperative hemorrhage , venous thromboembolism or thrombocytopenia between heparin and dalteparin . The results suggest that , given the small sample size of this trial , both drugs appear to be safe and the incidence of venous thromboembolism by postoperative screening duplex ultrasound appears to be low when these agents are used in combination with intermittent pneumatic compression devices In a prospect i ve , r and omized multicentre trial the efficacy and safety of the low molecular weight heparin ( LMWH ) fraction Fraxiparin ® and unfractionated calcium heparin ( Calciparin ® ) were compared for the prevention of postoperative deep vein thrombosis . Of 1909 patients included in the trial 1896 underwent abdominal surgery  and received either one daily subcutaneous injection of 7500 anti‐X a units Fraxiparin or 5000 units calcium heparin three times a day subcutaneously . Elastic compression stockings were worn by both groups of patients in the postoperative period . Before r and omization the patients were stratified in two subgroups with or without malignant disease . To assess the rate of deep vein thrombosis ( DVT ) , 125I‐labelled fibrinogen leg scanning was performed daily for 7 postoperative days . Positive results were confirmed by phlebography whenever possible . Venous thrombosis occurred in 27 of 960 patients ( 2.8 per cent ) given Fraxiparin and in 42 of 936 patients ( 4.5 per cent ) given calcium heparin ( P = 0.034 ) . The rates of proximal vein thrombosis were 0.4 per cent ( 4 patients ) and 1.4 per cent ( 13 patients ) respectively ( P<0.05 ) . Pulmonary embolism occurred in 2 of 960 patients ( 0.2 per cent ) treated with Fraxiparin and in 5 of 936 patients ( 0.5 per cent ) treated with calcium heparin . The two treatments were equally well tolerated . Intra‐ and postoperative blood loss , the number of wound haematomas as well as frequency and volume of transfusions were similar in both groups . The present trial demonstrates that a single daily subcutaneous injection of Fraxiparin is more effective than the established low dose subcutaneous heparin prophylaxis with 5000 units three times per day in preventing postoperative DVT after abdominal surgery in patients wearing compression stockings In this issue , we answer three questions with respect to loss to follow-up in a clinical trial : How important is loss to follow-up ? How is loss to follow-up calculated ? How many patients can be lost to follow-up without mistrusting the results ? 1 . How important is loss to follow-up ? The simple answer to this question is “ very important ” because loss to follow-up can severely compromise a study 's validity . Incomplete follow-up biases the results when either : The dropout rates are different between study groups ; or The patients who drop out are different from those who do not drop out . Why do these situations make a difference ? Because in each situation , those lost to follow-up often have a different prognosis than those who complete the study . For example , patients who receive treatment for cervical myelopathy may not return for follow-up because they became asymptomatic and felt no need to return to see the surgeon . Conversely , some patients may not return because they had a particularly bad outcome ( worse pain or function ) or complication , or because they died . In either case , bias can affect the validity of the inferences drawn from the study . 2 . How is loss to follow-up calculated ? There is much confusion about how to determine the proportion of patients lost to follow-up . In order to correctly calculate the follow-up rate , one needs to know the denominator . In a r and omized controlled trial ( RCT ) , the denominator for each group is the number of patients who were r and omized , not the number who received the treatment . For example , suppose we have an RCT comparing two treatment groups , Group A and Group B. The investigators evaluate 178 patients and r and omize 120 ; 61 to Group A and 59 to Group B ( Fig 1 ) . Following the figure , we note that 49 patients received treatment A and 52 received treatment B. At the final follow-up 40 were analyzed in Group A and 41 in Group B. How many were considered lost to follow-up ? Many would consider the loss to follow-up rate to be 9 ( 18 % ) of 49 in treatment A and 11 ( 21 % ) of 52 in treatment B using as the denominator only those that were treated . However , the real proportion lost to follow-up must consider those who were r and omly assigned , even if they did not receive treatment . In the present example , this is calculated as 21 ( 34 % ) of 61 for treatment A and 18 ( 31 % ) of 59 for treatment B. Fig 1 Hypothetical example of patients lost to follow-up in a r and omized controlled trial . When calculating loss to follow-up in a retrospective cohort study , all individuals receiving treatment during the study period should be used as the denominator , not just those with complete data . For example , let 's say you want to compare decompression plus lumbar fusion with decompression alone in disc herniation and the data available are all patients receiving either treatment in the last 5 years ( N = 275 ) . However , the data base from which the data are obtained is incomplete and only 190 have the necessary data available . Since the investigators stated as part of the inclusion criteria that only those patients with complete data are included , they consider the follow-up to be 100 % ( 190/190 ) . This conclusion is wrong . The denominator should include all patients who underwent the surgery irrespective of completeness of data . The follow-up rate for this example is 69 % ( 190/275 ) . 3 . How many patients can be lost to follow-up without mistrusting the results ? Some have suggested that 20 % poses serious threats to validity 1 . This may be a good rule of thumb , but keep in mind that even small proportions of patients lost to follow-up can cause significant bias 2 . One way to determine if loss to follow-up can seriously affect results is to assume a worst-case scenario with the missing data and look to see if the results would change . Here is an example : Let 's assume a multicenter study enrolled 500 patients into each arm of a study comparing artificial disc replacement ( ADR ) with fusion , and the end point is adjacent segment disease ( ASD ) . The trial numbers are found in Fig 2 . Fig 2 Hypothetical example of the effect of loss to follow-up considering a worst-case scenario . ADR 
    Output:
    
We found no difference between perioperative thromboprophylaxis with LMWH versus UFH in their effects on mortality , thromboembolic outcomes , major bleeding , or minor bleeding in patients with cancer .